Lukaku is shit so his numbers aren't irreplaceable thankfully, our top 6 fight is on
Lukaku scored 20+ a season fro three seasons running at Everton.
He then went to United, where Mourinho decided his pre-season training should involve eating as many pies as he could in 6 weeks. He scored 27 goals. United finished 2nd that season (behind Moneybags Mc spendalots) having finished 7th, 4th, 5th and 6th in previous seasons.
Last season, after the team came 2nd they rewarded the manager by not buying the CBs he wanted, like Harry Maguire. United started the season poorly and they brought in Jon Pertwee to see if he couldn't use his Dr Who skills to bring United back in time to when they were great. Seth Rogan gave one look at Lukaku and decided he wasn't Andy Cole and therefore there was no point to him. United go on a crazy run of games, as the players were run into the ground by the manager (and they fortuitously played a lock of middle to bottom sides). This period somehow persuaded the United fans that Lukaku was indeed shit and Rashford and Marial (though not as effective) clearly superior.
So Lukaku gets ostracised and pushed toward the door (but still manages to score 15 goals last season, which is pretty respectable). Just as the legs are about to fall off his team, OGS gets the biggest contract ever. He responds to this show of faith by leading his team to 8 points in 9 games, relegation form and ends up in 6th when a CL spot was on. He then decides that, indeed, Mourinho was right, they do need defenders and ends up buying... Harry Maguire. To finance this United sell Lukaku, their second top goal scorer and replace him with... oh!
Lukaku has got a rum deal here, but so have plenty of other United managers. Players a re brought in for a specific purpose and style, the manager gets shot for not being Alex Ferguson and the players he brought in also get shipped out. Its insanity. Don't blame Lukaku for the shitty direction at the top of your club.