I don't think he's a natural with the ball at his feet. Not a natural dribbler, especially from standstill. I doubt van Gaal encourages him to initiate attacks, but I don't think he has it in him to do so anyway.
Southampton's system meant that (when he wasn't passing it simple) he was largely carrying/running onto the ball into space. And they played at a high tempo. And their delivery in the final third was encouraged to be sharp and zippy. They seemed to play the final pass at the earliest opportunity once they crossed into the final third. Which is all the complete opposite to here under van Gaal.
So he's now being given the ball to feet, without much space to run into. And he's having a lot of time to think, whereas I think he's more of an instinctive attacker who's more useful against stretched defences, when his momentum's built.
I don't know if his dribbling will ever be a strong point (I don't think it can really improve that much, to be honest), but he can definitely improve his runs and movement off the ball in time.
He'll need to.