After scoring 27 goals in his debut campaign at Old Trafford following his £75 million ($96m) move from Everton, Lukaku began this season in equally fine form with four goals in his opening five games.
However, since scoring in a 2-1 win at Watford in mid September, Lukaku has failed to hit the back of the net in 10 matches in all competitions.
The 25-year-old was subsequently dropped to the bench for the 2-1 win against his former side Everton last month.
He was also named among the substitutes for the 3-1 Manchester derby defeat at the Etihad Stadium last time out, winning a penalty 30 seconds after coming on that was converted by Anthony Martial.
Lukaku’s struggles at Old Trafford are in sharp contrast to his form for Belgium, where he has scored four goals in his last five appearances.
That prompted Mourinho to fly to Brussels during the international break to try and find out why the striker was flourishing for his country but struggling for his club.
Mourinho declined a face-to-face meeting with Belgium boss Roberto Martinez, instead speaking to the Spaniard on the phone.
The Portuguese also watched Belgium’s dramatic 5-2 Nations League defeat to Switzerland – though he didn’t get to see Lukaku in action.
The former Anderlecht forward missed the game in Lucerne as well as the friendly against Iceland in order to undergo treatment on a hamstring problem that caused him to miss games against Bournemouth and Juventus earlier this month.
“I went, fundamentally, to be with Lukaku, to feel him, to see his situation, his injury,” Mourinho told reporters.
“Roberto invited me to meet him, which I decided not to because I don’t like to disturb people before matches, but I was with Roberto on the phone trying to feel, trying to understand the problem and evolution of the problem."
Lukaku has been passed fit for Manchester United’s game against Crystal Palace at Old Trafford on Saturday, though it remains to be seen whether he will be named in the starting XI or will once again have to settle for a place on the bench.