Rooney will return to United for the first time tomorrow since leaving to join Everton in the summer.
The ex-England and United captain won 16 major trophies during a glittering 13-year career at Old Trafford and is the club’s record goalscorer.
Mourinho spent £90m on replacing him with Lukaku and the Belgium ace has made an impressive start to life at United.
The big striker has plundered six goals in his first seven games to make sure Rooney isn’t missed.
But Mourinho is convinced Lukaku will struggle to emulate what Rooney did - no matter how hard he tries.
The United boss said: “Wayne arrived much earlier than Romelu did in terms of age.
“I think Wayne spent probably 10 years of his career in a different Premier League than you have now.
“A Premier League where it was easier to score goals, not just because of the difference between the top teams and others, but also because of the profile and the tactical interpretation of the competition.
“I think this Premier League is much more defensive minded, and much more difficult.”
Mourinho has hailed Rooney, 31, a United legend and insists he deserves a warm reception from home fans when he runs out at his stomping ground again.
But Mourinho has ordered United fans to also make life difficult for Rooney during the game.
He added: “I think he will get the welcome that he deserves.
"Sometimes in this country, the word ‘legend’ comes too easily. That’s not the case here. He is a real legend of this club.
“The number of appearances, the number of goals, the number of trophies.
"I think clearly he is one of the most important players in the history of Manchester United.
“And I think the stadium will show him the respect he deserves, I hope, before and after the match, but not during the match.”
Mourinho has confirmed Paul Pogba will be sidelined for several games with a damaged hamstring sustained early on into Tuesday’s 3-0 Champions League win over Basel.
But Mourinho says they have the midfield strength to cope without their star midfielder.
He added: “We have players and we have players waiting for an opportunity.
“Because we have started the season really well and with one match per week, with no need of resting, to make changes, we were basically going with the same players.
“But there are good players more than ready to play, so, honestly, we miss Pogba, we need Pogba, but we have good players.
“Herrera, Fellaini, Carrick, they are waiting - they are waiting for a real chance to start matches and to play matches from the beginning, and they are ready.
“So this is also our way of thinking. We lost important players last season during the season and during crucial moments of the season and we were not crying or making excuses about that. That’s just football.
“Squads are (there) to try to cope with these kind of situations and we know that for a few matches - I don’t know how many, I really cannot tell you how many I expect - but we are going to play without Pogba a few matches.”
