Lukaku, 24, joined the Red Devils in July, ending a four-season stay at Goodison Park, after the Belgium global scored 25 league goals last season and he has netted six in six in all competitions for United.
However, the retired England striker has since been dogged by an off-field controversy, while Everton have lost 3-0 in their last two games in the Premier League and the Europa League.
For most of the summer, it did look like the 24-year-old would be heading back to his first English club, Chelsea, while United's interest was seemingly in Alvaro Morata.
Former Manchester United striker Saha, who played alongside Rooney at Old Trafford during his spell at the Theatre of Dreams, is confident the Everton striker will leave his mark at Old Trafford once again but this time in blue on Sunday.More news: Neymar lawyer offers advice to help Sanchez leave Arsenal
"I think it is more hard now", Jose Mourinho stated.
The Red Devils signed three players in the summer transfer window. "I think this Premier League is much more defensive-minded, and much more hard".
Even so, Mourinho is of the view that Lukaku will continue to benefit from playing in a United side who carry more attacking threat than they did last season.
Speaking to the media, Mourinho spoke about the difficulty of the Premier League this season.More news: 'Morgan Tsvangirai is not dying'
"In the second season, you have your influence at that level".
"So it's not just the striker". I think that Romelu has a better team than Zlatan had. "We couldn't be happier".
He said: "I am not going to compare". I think it's very stupid to compare the incomparable. When players are so different, there is no point in comparing.More news: Mobile won't slow down the Apple Watch's data speeds after all
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