I don't think settling into the league has even been a problem for him, as he demonstrated earlier on in the season. The physicality or pace never seemed to be an issue for him. However, his biggest problem generates from the burglary incident, in my opinion. The period following the incident saw him struggle quite a bit, understandably as his mind never seemed to be focused on the game.
He's also been asked to play in far too many positions - RW, LW, AM, SS, CM. Haven't people realised what good consistency can do for both players and the team? Look no further than Rooney, Herrera, Young, Mata and Fellaini, if you want examples.