Yes lukaku has scored a decent amount of goals and generally, his output against the rest of the league has been good, as you would expect of a striker in the system we play. I'm not saying that part is disappointing, and generally not even the performances against those teams. Its the vast gap in both performances and his output against anyone decent. Of course, as you say, big games are always harder and it's dumb to expect anyone to score in them regularly or even perform in them regularly. Those who do are seen as the best players in the world typically.
The problem with Lukaku is just how bad he tends to play in them. I do think a large part of it is due to the system we play in these games, and as we see with Sanchez as well, Mourinho's style makes it nigh on impossible for anyone to play well sometimes, no matter how good they are. But these players at least show glimpses of their quality. Lukaku doesn't have the talent for me to perform in those games. Balls bounce off of him, he can't ever hold it up, and wanting him to create something out of nothing for himself or somebody elsewould be pretty hopeless as well. Everyone talks about service, but in the biggest games, to be one of the best, you want your striker to be capable of occasionally dragging you out of the mud with a moment of magic. Lukaku can't do that. I've said before he is a decent striker and in a typical Sir Alex side, he would probably just be a squad player or somebody who was used like Berba was. Against most teams, but then dropped in the bigger games to get a more complete all around team that can handle those games, because lukaku is useless in them. And that's my issue with him. He isn't good enough to have the status he has at the club, playing every single minute, never even being considered for rotation, never dropped even when out of form or playing games that doesn't suit him, never subbed out when playing poorly, and that's just because he's "big" so Mourinho uses him.
People might say it's harsh for me to compare him to our great strikers under Sir Alex, but that is what's needed at United to become an undisputed starting striker, especially in a one up top system when hes supposed to be the main man. Hughes, cantona, Yorke, Cole, van nistelrooy, Rooney, saha in 06/07, tevez in 07/08, van Persie... all talented strikers who also performed in the big games and played a large part in them, not only through their goals but also their overall performances. Everyone else was basically droppable based on who we played or if they were out of form. And it's not like we don't have options now... Sanchez showed he can score 30 goals playing as a striker for Arsenal last season, martial has shown his quality there often and potentially that might be his best position anyway, and then obviously Rashford as well who has huge potential there.
Like I said, I accept Lukaku has done well against the bottom half of the league. He's severely lacking against anyone decent though and the same issues come up every single big game. Can't hold up the ball. Can't battle in the air properly despite his size. Doesn't make enough or the right runs. Doesn't do anything special once he does get the ball to relieve some pressure. Is that really what you want from a striker you paid £75m for at 24/25 years of age to be your main striker for the next 5/6 years? Someone who can't perform in big games and becomes a liability in them? It's not just about the goals (which are woeful in those games). Its about how his overall game which is the bigger issue for me and has been since before he signed for United. He's a good player, but just not good enough for the level we want to reach basically. Just like Conte quickly realized Morata isn't good enough to be the main striker and Wenger realized Lacazette isn't good enough for that either. Lukaku has a much better goal record and is better then them, but still not there and it's evident in any big game.
There are so many things wrong with this.