Lukaku can be seen to have replaced Rooney at Old Trafford, with the Belgian joining from Everton as 31-year-old Rooney went the opposite way.
Lukaku joined Reds for £75m in July
Three Premier League games in, and Lukaku's netted thrice, as well as in United's defeat in the UEFA Super Cup to Real Madrid before the league campaign got underway. Rooney has praised the 24-year-old.
Speaking exclusively to talkSPORT, Rooney said that "for that United side," Lukaku is "the perfect player."
Lukaku will improve Man Utd, insists Rooney
"He is powerful, quick and strong and he scores goals, so certainly will improve that team."
Rooney's started his second spell at boyhood club Everton in good form, including scoring against United's rivals Manchester City in a 1-1 draw. He recently announced his international retirement, having already broken Sir Bobby Charlton's long-standing goalscoring record for country as well as, of course, for Man United.
Lukaku has some way to go to reach Rooney's heights at United, but has his backing in what has been a remarkably amicable departure from United for Rooney.
United to be big success, says former-captain Rooney
In the same interview, he backed United, not just Lukaku, to continue improving. The Reds won both of their first two league games 4-0 before a 2-0 victory against Leicester City. Top of the league going into the international break, United look exciting and well balanced as a team.
"They will be a big success," Rooney said, with the Englishman set to face his former side at Old Trafford for the first time in mid-September.