I think he'll be world class under LvG; out of his comfort zone (which seems like he was, under Fergie), and under guidance of good manager.
He is soon to be 24 years old, at this point he should be working on the smaller things in his game, not try to develop a huge amount. He struggles with positioning, body control, first touch, finishing and anticipation (movement in the box mainly, never starts a run before the pass is made and fail to attack dangerous space). Take a look at our players like RVP, Rooney and Mata, then think of Welbeck, he has so much less quality and it's so much it just can't be made up for at this point. Look at a player like Januzaj that is five years younger, he has far less to improve on and much better at most aspects of the game already.
This sounds very negative, but I actually would not mind if he stays. He is local and he would be ok as fourth or maybe even third choice as a forward. He will also find it harder to get games, because we will buy new players and they will be great players I suspect. We will also have to judge him against our aims as a club, would we achive our goals of winning the PL and CL with him as a trusted forward? Compared to the players that elevate the top clubs to the top of Europe often thend to be a lot better than him or has one or two extreme qualities that he just don't have at this point.
What is a big concern is that he has actually expressed, trough his agent, that he wants to leave to get more football. Then he will just have to move on for me because his focus is off and that is not the attitude he should have if he wants to improve and make it big at United.
To think he will become world class, when we argue if RVP, Rooney and Mata is at that level is more a dream than a serious objective judgment of the player. He is far from what I hoped for after his youth career and his few appearances as a 17/18 year old. He is sadly not top class at anything, but I still believe he can improve and be a decent squad player. But, then he have to put his head down and work, not feel that he deserve to play before players like RVP and Rooney at this point, that should not be his focus at this point.