"I think clearly he is one of the most important players in the history of Manchester United".
But Mourinho still thinks Rooney - who notched a record 253 goals during his 13 years with the Red Devils - has plenty to offer in the top flight.
Attack: Mourinho seems to be regularly rotating Anthony Martial and Marcus Rashford on the left side so we could see the England worldwide recalled on Sunday.
"It's not just about the striker", Mourinho said.
Lukaku faces former club Everton this weekend having netted six goals in as many matches in all competitions for United since his £75million move. "I think sometimes in this country, the word "legend" is used too easy".
Similar things were said about the returning Wayne Rooney: "I think that he will get the welcome he deserves".
Everton manager Ronald Koeman has also spoke out about the return of Rooney.
Pakistan celebrate return of global cricket with memorable win over World XI
His and Pakistan's only hundred in a Twenty20 worldwide had come against Bangladesh during the World Twenty20 tournament in 2014. Came to chase down the total, World XI never looked in the contention as they lost their batsmen at regular intervals.
On the field this season his early form was promising too, and before he announced his global retirement we saw flashes of the old Rooney, scoring goals and proving the doubters wrong along the way.
Everton have been a bit inconsistent this season, as they have mixed some impressive displays with some really poor ones.
Koeman has said he is looking for the senior members of his squad, like Rooney, captain Phil Jagielka and Ashley Williams, to turn their fortunes around. The way the team plays, they have to.
United boss Jose Mourinho intends to bring the pair back in central defence on Sunday.
Paul Pogba's hamstring injury against Basel means he could be out for six weeks which puts Herrera, Fellaini or Michael Carrick in contention to take over.
Luke Shaw, on the other hand, is fully recovered from his injury and is available for selection if the manager wants him. "When a team doesn't create a lot, when a team doesn't have the ball, when a team is just reactive, it is more hard for these players but when the team is in control and makes decisions on the direction of a game, when the team is so offensive, when the team puts so many people in attacking areas, I think they have to score goals".
Historically, it has been a classic fixture. Koeman has added some very good players, but adding proper replacement of Lukaku can been seen as a failure.