The 24-year-old has scored just once in last 10 games but Mourinho believes he offers much more than goals on the pitch, inspiring teammates to follow his lead and work harder.
"In the last minute he (Lukaku) was fighting and running back like he was in the first minute, so I think it is also a mentality question," Mourinho told reporters.
"The way Romelu wins the corner for the goal, the way Romelu ends the game-making tackles in the left-back position - that is the mentality I want.
"Some guys are capable to go into the limits of their efforts and some other guys even with a lot of talent, they don't manage to do that. So Romelu, for me, (is) fantastic. Didn't score, I don't care."
Mourinho's side, currently second in the league, remain eight points behind leaders Manchester City, who are unbeaten with 37 points after 13 matches.
United next visit Watford, who are eighth on the league table after an impressive 3-0 win at Newcastle United on Saturday.
Ahead of their trip on Tuesday, Mourinho said he was still upset about the manner in which the game was officiated when United were beaten 3-1 at Vicarage Road last season.
"Against Watford I was very upset with Watford's first goal after some guy sent (Anthony) Martial to the hospital and the referee let it go, that is the reason why I was very, very upset at Watford," he added.
"They have a squad full of very good players who are not normal to be in these mid-table teams. They have a good coach and have improved a lot from last season."
(Reporting by Hardik Vyas in Bengaluru; editing by Amlan Chakraborty)
