Lukaku has come under fire for a goal drought of late but added to his goal against Bournemouth on Wednesday with the opener against West Brom.
An Ahmed Hegazi own goal put United 2-0 up at half time and on cruise control.
Gareth Barry pulled one back for the Baggies – prodding home from a corner.
United ran out victors but it was Lukaku’s lack of celebration for his goal against his former club that could be of concern.
The Belgium international headed home on 27 minutes but instead of celebrating simply walked to thank assist-maker Marcus Rashford.
Asked about Lukaku’s reaction to his goal, Mourinho said: “I didn’t celebrate too.
“Nobody was happier than me.
“Fifteen years ago I look at myself celebrating goals. With maturity you go into levels that you control your emotions better.
“Romelu was a happy kid at West Brom [during the 2012-13 campaign] – maybe at the back of the mind it was his love for the club.”
Some fans have called for Jose Mourinho to drop the forward in place of Zlatan Ibrahimovic, who has recovered from a knee injury.
But Mourinho reaped the rewards of keeping Lukaku in his starting XI today.
Full match report to follow…