As I posted somewhere else, Lukaku has fairly good stats in the Premier League. The real issue is Lukaku's weight. While he's always been a proper unit, he has gained a lot of weight the past 1-2 seasons, which makes him too heavy and static, and limits his (already limited) techinical abilities even more.
If he could shred 5-10 musclemass and get back to the weight of him Everton and primo United years, then I'd take him in a heartbeat.
In regards to his stats against bigger sides - who cares? If we beat all the smaller teams 9/10 times, and we were only falling short because of missed points against the likes of Juventus, Roma, Napoli etc., then it would've been a problem, but we're faaaaar too often failing against smaller teams, and could actually use a certified-bomber against the mastodonts that is Lecce and Spal.