I have a feeling that it has to do with the fact that after the successful run we had with the system, we couldn't seem to win a game to save our lives (remember, after the City match, we should've been able to reasonably finish either third or second). The system, while being quite effective against teams willing to press United, did not seem to work against teams more content with sitting back against us. Now, regarding the 4-2-3-1, it does work, on some level, but is far more defensive than the other 4-3-3. I believe the line of thought is to mitigate our defensive frailties by dropping in 3 defensive midfielders, while believing that our front 4 have the capabilities to unlock a defense with their individual skill, often aided by overlapping fullbacks (someone like Valencia or Shaw would be golden in this system). It does, however, suffer from the same issues that the 4-3-3 did in having trouble in breaking down teams that park the bus, but I imagine the thinking here is that the sheer quantity of attacking should eventually result in a defensive error that unlocks the first goal. Unfortunately, a fair number of players are now simply bereft of confidence after a start to the season which, honestly, was fairly strong - United were leading the league and were in and about the title race up until we drew at home to PSV Eindhoven, a match which kicked off our horrendous run throughout December and is something which hasn't really been solved till now. To me, at least, it was as though a switch had been flicked which suddenly caused the team to start playing much worse than they had been earlier.