Not a Lukaku fan by any means, and was against signing him last summer, but he's a small fraction of the overall conundrum under current management, and at times cops a disproportional amount of flak (a lot of that indignation should be directed elsewhere). We knew who he was and what he offers or will offer in terms of his overall skillset after watching him for West Brom and Everton and the Belgian national team — he's someone who will score a fair amount of goals over the course of a season or qualifying campaign and help dispatch the opposition when the team is consistently creating a good amount of chances, keep the opposition centerbacks engaged to an extent, offer a mixture of mobility and stature, and work reasonably hard for the team (though he will have his ups, and downs where the issues are more pronounced) — he's not the most technically proficient, doesn't always perform when the going gets tough against higher caliber opposition (translates to Belgium as well), doesn't fit the profile of a traditional target man in terms of his hold-up or combination play, and there's a ceiling to what he does (which will impede him from reaching the elite tier) — all of that wasn't going to change on a fundamental level for someone who's in his mid 20s and has been playing at a first team level for close to a decade.
It's not his fault the management unwittingly envisions him as the second coming of Drogba or Costa in terms of what he's asked to do (even though he has more in common with someone like Higuaín, albeit weaker in terms of technique and less clinical, or perhaps McCarthy who won the Champions League at Porto under José), or that he's a seemingly immovable fixture in the starting XI, or that the attacking habitat around him isn't structured well enough and is in a perpetual state of flux, or that our combination play and overall attacking movement in systemic terms is below par — which makes it harder for the players to occupy good spaces and amplifies their individual flaws, or that United made him the second most expensive Premier League signing of all time — all of that is on the management. Almost every striker has days where their performance is woeful — Lukaku will be alright on most days and has already scored 4 in the league, and labeling him a donkey on a regular basis or treating him as a piñata to expend a sense of collective frustration (which is what happens in this performance thread) is unbecoming and unproductive. As an organisation United needs to make sure he's a bandage — a transitional player between eras when the club is fortifying or rebuilding (like Džeko for City, Gómez for Bayern though they were admittedly in a much better state than us) — and not the spearhead for the long haul, so the broader term onus is very much on the management and upper-tier decision-makers.