He is not good enough to be the type of forward we need. He needs to relax, and play with a poise, focus and class which befits the shirt. That is not just a preferance, but a technical requirement to be the main man at a club like United. You need to have that confidence of the club, and Lukaku has not grown into that. In fact, he has gone the other way. Anyone who plays 100 games as No 9 for United will score his fair share of goals. As Rio stated, you have to do more than that though... Cole had incredible work-rate, commitment and speed. Van Nistelrooy was not interested in much outside the box, but was almost the perfect goal-scorer and cared for little else. Mark Hughes was an elegant, superb technician, and very, very tough guy to play against, natural match-winner who played against whoever, wherever with class. Van Persie was a technically gifted player, hungry for success, who didn't miss in front of goal. Rooney was simply brilliant, all round. Zlatan absorbed all that was around him majestically and scored for fun. Ronaldo is simply Ronaldo.
So, all quite different, all played No 9 at one stage or another and all wore that shirt/position with ease. The fans, their team-mates, the manager, the whole club, trusted them without hesitation.
Lukaku simply does not provide this. Add to this him putting on much weight (and I still do not see why....), and the nonsense about after-shave etc.. doesn't help him.
Apparently Lukaku is a very clever (educated) guy, and is fascinating person to talk to... but his game-brain seems a little 'slow', and this is a problem. Compare Rooney, always as bright as a button on the park.
Lukaku will score goals anywhere he goes. But he hasn't done enough to really define himself as the No 9 for United.
A good Lukaku could be Man United captain. Lead the team, keep Pogba focused and smash the hell out of opposing defenders. But he simply has not seemed to be able to show this side of himself. A shame, but if we can get top money, let's take it, and get someone hungry to be a top forward at a big club.