"He's got enough goals, perhaps not enough goals against those big boys but enough to justify the money that was laid out for him," he told Sky Sports.
"However, with Ibrahimovic coming back it does put that little bit of extra pressure on him so last night's goal was important because then he continued that run, if not then there's pressure on Jose to put Ibrahimovic back in that team after what he achieved last season."
Mourinho will be hoping Lukaku is on target when United face rivals Manchester City on Sunday.
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Jose Mourinho splashed out £75m on Manchester United striker Romelu Lukaku in July
Emenalo hints Conte was disappointed in missing out on Lukaku
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Romelu Lukaku has scored 12 goals in 22 appearances for Manchester United this season
The Red Devils are currently second in the Premier League, behind the Blues.
But they’re already eight points adrift of Pep Guardiola’s side.
And that difference means United could see their title hopes dashed if they fail to beat City.