Romelu Lukaku was a bad fit for Manchester United. He was an incredibly talented forward, but he was not a target man and that was the role that Jose Mourinho hoped he could operate in during his tenure.
As United fans will remember, the Belgian's close control left a lot to be desired and this led him to be ridiculed by rival fans and eventually the Old Trafford faithful. United fans were happy to see the back of Lukaku when he departed for Inter Milan in a deal that saw the club lose some money, but not a substantial amount.
Of course, when Lukaku reached Milan and finally played under Antonio Conte who had been chasing him since his Chelsea days, he became one of the most unstoppable forwards in world football. In 95 games he scored 64 goals and registered 16 assists.