The one thing he had going for him in my eyes was his movement, he seemed to often find himself at the right place and at the right time, but his finishing would let him down. Without that movement, all he has left right now is just hustle, which even that is taking a hit because he's clearly not in top shape.
I've always thought he's gotten by on his first 6 months at the club, until defenses got a read on him and adjusted. Since then, he's had moments here and there, but he's clearly a limited player in ability and impact. If his pace doesn't take him past his defender, he has very little else to fall back on. Yeah, he'll have the odd moment of skill that pays off and gets him past his marker, but they are so rare and far in between, it's not something you can consider an asset in his game.
None of this is really his fault though, he has been thrust into a side that doesn't have much of an alternative. This is a player that would be a bit part player in the golden era of United, a feel-good story of another lad from Manchester making some sub impact from the bench and he would be integrated into a team where the pressure isn't on him to make things happen, but rather any positive contribution is a cherry on top. He wouldn't be in the bargaining position to get a contract he should never be even thinking about, or wearing the #10 which might give him the impression he's already made it.
If he can re-evaluate his game, and find a way to make an impact from the RW, he could share the duty between him and Greenwood supporting Martial up front and whoever else might come in at striker in the near future (hopefully). He can work on his crosses, which at times are very good, and he can always pop in for the odd goal here and there. He can prove me wrong in a few years, but right now I just can't see him as someone who will be a 20+ league goal scorer.