He won't improve on those aspects though. He is what he is. He's not going to develop a top class first touch or better close control.
The only thing that will improve from here on out is the mental side of his game: his tactical intelligence, his composure and decision making.
United are spending £100m for him because he's as close to a guaranteed 20 goal a season striker as there is in the Premier League. They're not taking a punt in any way, they know exactly what he is and what he isn't - Chelsea know it too. He'll rattle goals galore for the scumbags, I don't doubt it for a second. Will he be able to score against the best sides in the Premier League and Champions League, consistently make the difference in those games? No, I don't think he will. He's just not that good.
I also personally think he's as good as he'll ever be because he doesn't strike me as in any way intelligent when he plays, or when he talks, and I think that limits what you can improve on as you get older. He's also as fit and as fast as he's ever going to be too. Pretty soon he'll have played at the top long enough where he's nursing a persistent hamstring injury or has a dodgy knee and so on. Never playing fully fit.
All that said, he is a good addition to their team and much needed now that they've lost Ibrahimovic. He's more likely to score 25 League goals than all but one striker in the League next season, perhaps more likely than mouths-a-gaping now that he'll have more walkovers than he got last season at Everton.
He's much more of a sure thing than Lacazette, let's put it that way. Cost aside, he's comfortably the better signing of the two.