Thanks for reading everyone. I appreciate those who tried to let me down gently.
I'd like to try defending it at least once before moving on. I should clarify that I'm not going to argue against the consensus that AWB would not make a good defending midfielder. So if you think that should be enough to settle the matter then fair enough. After all, it is against common sense to play someone in a role for which they are unsuited, not to mention it does operate with a bit of wonkiness. I cannot reward your patience if you read this.
The biggest flaws pointed out so far are, in order of their resonance:
1. AWB cannot pass the ball;
2. AWB has poor positioning;
3. AWB has no close control; and
4. Inverted role would expose right flank.
Again, I don't disagree with these, with the possible exception of (3). I think he is more comfortable on the ball than he's given credit for, but that's not a hill on which I'm willing to die. So briefly, I think he has an okay rate of getting out of trouble when cornered near the touchline, and that he is not entirely to blame for receiving the ball in bad situations in the first place. In any event, enough people have made the opposite point to make me reconsider this.
As for exposure on the right, it is certainly a problem - but one that is already happening. AWB already has to recover from the opposition by-line every time he joins the attack, and we already lack cover to compensate for this.
The difference is that instead of recovering from the end of the pitch, he would be recovering from the base of midfield. The distance is shorter, but he also has less time to do it. Overall, I suppose there is a fair chance that the exposure could be more exploitable as admitted in the OP.
Whether that is worth the potential risk depends on two things: (1) If a counter attack is on (for the opposition), and we are open both in midfield and at right back, is defending direct balls towards the latter preferable to being run over in the former? and (2) If we are dealing with both exposures simultaneously, and we can place a man in either area, which option would be more effective?
Question (1) is a bit of an over-simplification, but I think the general point holds, and the answer is "yes". The sideline is friendly when it comes to long balls and it limits the succeeding options.
Question (2) depends on whether AWB can defend effectively in midfield. So this is where his positioning becomes problematic.
Certainly I think he would be terrible if he's asked to sit in front of the defence and cut out passing options. He can't do what Makélélé or Carrick (in his later years) did. I'd like to think that's not what I implied in the OP either.
The test should be whether he is sharp enough to closely engage opponents as soon as we lose the ball, and then either attempt dispossession or delay until the real midfielders recover. Positioning isn't unimportant here, but I think we are imagining how bad AWB is if we think he'd be a net-negative here.
Remember, our midfield will include at least one of Fred, Pogba, Matic (whose positioning isn't bad, but just lacks the mobility to actualise it) and van de Beek, so the alternative isn't fielding a defensively sound midfielder, it's fielding a defensively poor midfielder with no rear cover.
Here's what will likely happen: With Rashford returning and Sancho improving, we are going to see Pogba in a double-pivot again. After a few games, we are going to complain it doesn't work and we need the solidity of McFred. So we will revert to that for a while. And a few toothless displays later people are going to urge Ole to be more attacking, and back to Pogba in midfield we go, and the cycle continues.
So if Pogba in midfield is inevitable, are we worse off by relying on his wanting defensive discipline, or by providing him with an imperfect transitional cover? The problem, if you've made it this far, is having to speculate whether that cover will be useful 90% of the time, or 10% of the time. Obviously, I am saying that despite his poor positioning, his aggression will be useful enough to outweigh the risks.
Now on to the most difficult part to reconcile with: AWB's passing. I don't want to re-use the argument that this is something already happening (i.e. his poor passing is already costing us offensively). For one thing, it would be bad faith: poor passing in the final third or out wide in our own half is less dangerous than poor passing in midfield; for another, I realise I am less harsh a critic on his wing-play than some on here (even though that's the
raison d'être for this idea).
You might find it laughable if you read the OP that this scheme is meant to primarily improve the attack, and only secondarily the defence. Here was the thinking: The double-pivot will likely always feature one of Fred/Pogba, who will carry the creative responsibility (presumably over McTominay). For Fred to be more useful, he needs to occupy the space between the upper and lower block; for Pogba to be more useful, he needs freedom and more than one person covering him.
Theoretically, having AWB invert his runs when we are trying to exit the transitional phase creates 4v3 situations in midfield. I had hoped that calling it a "defensive inverted fullback" in the title would distinguish it from the role that Pep employs at Man City or that Shaw occasionally undertakes for us. The job description in the OP was "do not join the attack" and "recycle possession with quick, unambitious passes".
Of course, it still takes a great passer to do this well to the degree of a Busquets or a Carrick. But we are not looking for excellence, we are looking for better than what we currently do, and for-the-rest-of-the-season will, have. What AWB would have are Maguire and Varane behind him, Shaw and McTominay/Matic next to him, Fernandes and Pogba/Fred ahead of him, and a numerical advantage.
None of this matters if he doesn't have the technique to be at least competent, but I'd argue AWB isn't that kind of bad passer. His flaw is his inability to see beyond the obvious and the routine. His passes hurts our attacking momentum, but are relatively accurate considering whence they are made. Other than final balls, he most frequently gives away possession when he is isolated and surrounded near the touchline. As I said, I don't think he is solely to blame for being in those situations.
Ironically,
WhoScored currently describes passing and ball-holding as his strengths so far this season, although I'm the first to concede it doesn't truly reflect his ability apropos the former. (It cites his interception skills too, but again, I'm not attaching too much weight to it.)
The point is, if asked to be a simple water-carrier, I argue that he would be tolerable enough despite his passing. He will waste possession more often than (some, though seemingly if harshly not ours) natural midfielders, but it is worth the risk if we can turn Fernandes into a free man with relative frequency and get Pogba/Fred to play in a more natural position.
I know that someone commented they stopped reading as soon as they saw Fred's name - and I get the frustration - but what we have is what we have. We cannot already be thinking about the summer window as solution.
Simultaneously, we shouldn't be comparing it to what Man City's inverted wingbacks do. They form arguably the best midfield in the world and we struggle to keep the ball against Aston Villa. Our requirements are very different.
To sum up, here's what I'm trying to say: Can the team attack better with an additional midfielder instead of an attacking fullback? Yes, despite AWB's passing. Is the team harder to hit on the break with the defender recovering from midfield instead of the bylines? Yes, despite his positioning. Is the right side more exploitable with an inverted fullback instead of an attacking fullback? I don't know, but I think the difference is small.
Because of the two big "despite"s, I don't think too many will be persuaded by this follow up. But to paraphrase a bit of Biden-ism: don't compare it to the Almighty, compare it to the alternative.