I think this is an interesting thread, that has stimulated a very interesting and pretty complex discussion. Immediately, I'm not taken with the idea that defensive midfielders are, given whatever tactical and stylistic developments, particularly well suited to moving to or filling in at full-back; or that defensive midfielders are the most tactically versatile players around.
I think some sort of definition of or consensus towards the type of midfielder we are talking about would obviously be useful. Are we talking defensive midfielders generally; and to what extent can what we might call defensive midfielders and what we might call central midfielders be equated at the moment? My feeling is that most midfielders playing centrally in the big leagues today are sort of hybrid central-midfielders/defensive-midfielders - I think there are relatively few around who really penetrate into attacking positions in and about the opposition team's area, and that their primary objective is defensive in terms of the amount of ground they cover, their pressuring of opponents, their tactical discipline, and their focus on retaining possession. My impression is that there has been a slight decline in the number of purely destructive defensive midfielders about - in the sort of player who would perhaps cover less ground, sit entirely in front of the back line, and sweep up and make tackles (and fouls) in that position without having to recycle the ball. Which is to say that I think there is something to the notation 'CM/DM'; that this does serve to indicate the sort of hybrid player a modern central midfielder is. At the same time, whilst I think all 'CM/DM's do need to be able to recycle the ball and retain possession, still there are some midfielders who are expected to pass the ball a bit more creatively, with more range, as the first step towards building attacks.
In so far as both defensive midfield/central midfield and full-back roles require lots of running, a degree of comfort on the ball, and tactical discipline, there are similarities. For me the sort of tactical discipline required of a defensive/central midfielder is quite different from the tactical discipline required of a full-back who, like abundance says, has to be able to work as part of a defensive line and read diagonal runs. I also think there's a knack to playing in a wide position and a knack, or an understanding required to play centrally; and perhaps it is arguable and not entirely archaic to suggest that a full-back needs to be able to play a long pass or a cross less than a short pass, whereas the opposite is the case for a midfielder.
I suppose ultimately I don't think a generalisation can usefully be made: I don't think it is right to say that there is, as things stand, a close correspondence allowing defensive/central midfielders to occupy full-back roles, but a growing discrepancy between the roles of full-back and centre-back. I think the positions are still fairly specialised. As I don't agree that defensive/central midfielders are especially tactically flexible, I think it's just as arguable to suggest that many full-backs would actually make decent defensive/central midfielders. Again, however, just as the role of full-back requires certain abilities defensively and offensively which I don't think the defensive/central midfielder necessarily possesses, I do think you need one midfielder who can instigate moves with concise, clever, perhaps slightly more expansive passing - so that a full-back filling in for a midfielder may do a good job, but probably only alongside a specialist player.
Looking for any examples and indications of patterns in my own team and in the world at large: though Newcastle have three very capable defensive/central midfielders in Cabaye, Guthrie and Tiote, I wouldn't fancy any of those at full-back, whereas I think Santon, or in recent years Enrique, perhaps could adequately occupy a midfield role alongside a specialist player. On the other hand, Steven Taylor, who started and has played most of his career at centre-back, was one of our best players playing at right-back in our relegation season. He played that role and actually offered much more to the attack than Habib Beye, a very good player for us, but who was predominantly defensive in outlook. Elsewhere, there seem to me equally numerous (though actually fairly few, and often limited to the experiments of the bigger clubs who can afford to experiment) and equally potent examples of players comfortable at full-back and midfield (Hargreaves, Flamini); at centre-back and full-back (Ramos, Chiellini); and in midfield and at centre-back (Mascherano, Mbia, and Manchester United attempted this with Carrick at points this season).
I suppose a lot of this suggests a 4-4-2 or a relatively static 3-5-2 or 4-3-3. Bielsa's system at Chile, from what I understand, really operated within different parameters, with 'wing-backs' sometimes spreading wide to attack but then tucking in to defend, and changing positions with the two wider 'centre-backs' in a three-man back-line. That all seems to eschew the notion of 'position' to such a degree that I'm not sure it relates easily to the discussion, or allows any conclusion about the versatility of 'defensive midfielders'.
Another point which maybe relates tangentially to the debate is that it seems to me there's often talk about full-backs and wingers or wide-midfielders switching positions - suggestions that a certain full-back may be better on the wing, or that a wide-midfielder will be most suited to the full-back position in the long term - but such switches infrequently actually happen. Am I simply missing a host of instances? Or is it that some attacking full-backs (like Ramos, Maicon, Alves prior to Barcelona's tactical modifications this season), possessing also plenty of stamina, are simply capable enough and allowed to navigate the whole of their side of the pitch without a positional change being necessary? I can think of Abate as an example of a winger who has been moved to full-back, but I can't think of many examples, whereas in England people have often suggested, for instance, Bale, Johnson, Enrique, Rafael switching position, but it has never occurred.
I think some sort of definition of or consensus towards the type of midfielder we are talking about would obviously be useful. Are we talking defensive midfielders generally; and to what extent can what we might call defensive midfielders and what we might call central midfielders be equated at the moment? My feeling is that most midfielders playing centrally in the big leagues today are sort of hybrid central-midfielders/defensive-midfielders - I think there are relatively few around who really penetrate into attacking positions in and about the opposition team's area, and that their primary objective is defensive in terms of the amount of ground they cover, their pressuring of opponents, their tactical discipline, and their focus on retaining possession. My impression is that there has been a slight decline in the number of purely destructive defensive midfielders about - in the sort of player who would perhaps cover less ground, sit entirely in front of the back line, and sweep up and make tackles (and fouls) in that position without having to recycle the ball. Which is to say that I think there is something to the notation 'CM/DM'; that this does serve to indicate the sort of hybrid player a modern central midfielder is. At the same time, whilst I think all 'CM/DM's do need to be able to recycle the ball and retain possession, still there are some midfielders who are expected to pass the ball a bit more creatively, with more range, as the first step towards building attacks.
In so far as both defensive midfield/central midfield and full-back roles require lots of running, a degree of comfort on the ball, and tactical discipline, there are similarities. For me the sort of tactical discipline required of a defensive/central midfielder is quite different from the tactical discipline required of a full-back who, like abundance says, has to be able to work as part of a defensive line and read diagonal runs. I also think there's a knack to playing in a wide position and a knack, or an understanding required to play centrally; and perhaps it is arguable and not entirely archaic to suggest that a full-back needs to be able to play a long pass or a cross less than a short pass, whereas the opposite is the case for a midfielder.
I suppose ultimately I don't think a generalisation can usefully be made: I don't think it is right to say that there is, as things stand, a close correspondence allowing defensive/central midfielders to occupy full-back roles, but a growing discrepancy between the roles of full-back and centre-back. I think the positions are still fairly specialised. As I don't agree that defensive/central midfielders are especially tactically flexible, I think it's just as arguable to suggest that many full-backs would actually make decent defensive/central midfielders. Again, however, just as the role of full-back requires certain abilities defensively and offensively which I don't think the defensive/central midfielder necessarily possesses, I do think you need one midfielder who can instigate moves with concise, clever, perhaps slightly more expansive passing - so that a full-back filling in for a midfielder may do a good job, but probably only alongside a specialist player.
Looking for any examples and indications of patterns in my own team and in the world at large: though Newcastle have three very capable defensive/central midfielders in Cabaye, Guthrie and Tiote, I wouldn't fancy any of those at full-back, whereas I think Santon, or in recent years Enrique, perhaps could adequately occupy a midfield role alongside a specialist player. On the other hand, Steven Taylor, who started and has played most of his career at centre-back, was one of our best players playing at right-back in our relegation season. He played that role and actually offered much more to the attack than Habib Beye, a very good player for us, but who was predominantly defensive in outlook. Elsewhere, there seem to me equally numerous (though actually fairly few, and often limited to the experiments of the bigger clubs who can afford to experiment) and equally potent examples of players comfortable at full-back and midfield (Hargreaves, Flamini); at centre-back and full-back (Ramos, Chiellini); and in midfield and at centre-back (Mascherano, Mbia, and Manchester United attempted this with Carrick at points this season).
I suppose a lot of this suggests a 4-4-2 or a relatively static 3-5-2 or 4-3-3. Bielsa's system at Chile, from what I understand, really operated within different parameters, with 'wing-backs' sometimes spreading wide to attack but then tucking in to defend, and changing positions with the two wider 'centre-backs' in a three-man back-line. That all seems to eschew the notion of 'position' to such a degree that I'm not sure it relates easily to the discussion, or allows any conclusion about the versatility of 'defensive midfielders'.
Another point which maybe relates tangentially to the debate is that it seems to me there's often talk about full-backs and wingers or wide-midfielders switching positions - suggestions that a certain full-back may be better on the wing, or that a wide-midfielder will be most suited to the full-back position in the long term - but such switches infrequently actually happen. Am I simply missing a host of instances? Or is it that some attacking full-backs (like Ramos, Maicon, Alves prior to Barcelona's tactical modifications this season), possessing also plenty of stamina, are simply capable enough and allowed to navigate the whole of their side of the pitch without a positional change being necessary? I can think of Abate as an example of a winger who has been moved to full-back, but I can't think of many examples, whereas in England people have often suggested, for instance, Bale, Johnson, Enrique, Rafael switching position, but it has never occurred.