The Belgian scored a career-best 25 goals from 37 Premier League appearances for the Toffees last season, and has four already this season for the Red Devils.
“I don’t say impossible but that was close to impossible to get someone in who will score that amount of goals that Romelu scored for Everton,” the Dutchman said, via the Liverpool Echo.
“With Romelu we had enough fights about how he needs to press, how he needs to run, how he needs to move but in the box, with opportunities to score, he is one of the best.
“It is always difficult to replace players like him because of the person he was, the physical boy that he is, the number of goals that he scored. It is difficult.”
Lukaku’s No. 10 shirt at the club has been taken up by Wayne Rooney, who has scored twice for the Toffees this campaign having gotten five for United last season.