Lukaku made a big-money move from Everton this month, while Rooney went back to his roots in re-signing for the Merseysiders after 13 years at Old Trafford, during which he became United's record goalscorer.
Saturday's 5-2 win over the Galaxy offered United fans a glimpse of the future, with Mourinho insisting Lukaku brings an entirely different set of attributes to the table from Rooney.
"To compare Rooney and Lukaku is incomparable and Rooney for us was not a striker. Lukaku is more a target man with his back to the opponents," the Portuguese explained.
"It's a point of attack for everyone who has the ball in defensive lines, he's really fast and a good thing for me today is he showed he's a team player and was not worried about himself, he was worried about the improvement of the team."
It was United youth product Marcus Rashford who stole the limelight at StubHub Center with two goals, while Marouane Fellaini, Henrikh Mkhitaryan and Anthony Martial were also on target.