The £75m summer purchase has a respectable 20 goals in 38 appearances overall for the Red Devils since arriving from Everton, while also helping out with five assists in the top flight.
Lukaku's form in front of goal is less impressive when taking into account games against the current top-eight teams, though, having failed to score in 10 matches versus Manchester City, Liverpool, Chelsea, Tottenham Hotspur, Arsenal, Burnley and Leicester City.
In fact, the Belgium international has yet to register a shot on target against teams in the top six, but Mourinho is happy enough with the striker's showings so far in a United shirt.
"I am not worried - 21 goals is not bad at all," he told reporters ahead of Sunday's home game against one of those teams in Chelsea.
"I don't measure the players' performances the same way you do. I have other points of analysis and I am happy with Romelu's performances, even when he doesn't score goals."