Lukaku has already more than justified his £75million price tag following his arrival from Premier League rivals Everton in the off-season, netting 10 goals in nine games.
Anthony Martial and Henrikh Mkhitaryan were also on the scoresheet, and Mourinho was delighted to see the continuation of Lukaku's sublime form.
"He is having a great record but we know he is a very good player who will score lots of goal when surrounded by quality players," Mourinho told BT Sport .
"But I have to admit he is scoring important goals in almost every game."
The result leaves United with six points from their opening two Group A fixtures following a 3-0 win over Basel at Old Trafford, much to the delight of Mourinho.
"The important thing is two matches, six points and a good position," he added. "We have four more matches to play and have started really strong."
The emphasis on team was evident in Lukaku's post-match comments, with the striker refusing to be drawn on goal targets.
"I just want to win trophies, that's it," he said.