Romelu Lukaku was almost impossible to replace at Everton, according to manager Ronald Koeman.

Lukaku, 24, joined Manchester United in July, ending a four-season stay at Goodison Park.

The Belgium international scored 25 league goals last season, and he has netted six in six in all competitions for United.

Koeman, whose team has made a slow start to the campaign and visits United on Monday (AEST), said replacing Lukaku was always going to be a huge ask.

"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.

"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."

Everton made numerous signings, adding the likes of Wayne Rooney, Gylfi Sigurdsson, Davy Klaassen and Sandro Ramirez.

However, it has scored just two goals in its first four league games, and was humbled 3-0 by Atalanta in the UEFA Europa League during the week.