Lukaku arrived in the summer from Everton.
Duncan Castles, of the Daily Record, told the Transfer Window Podcast: “I made a point of asking Mourinho's guys just after they got him in America and had him for a few weeks training there what the difference was between the Lukaku they'd worked with at Chelsea.
“They were very clear he was a completely changed individual.
“Much more mature. They thought a better, more mature footballer.
“The product they got on the training field after purchasing him for £75m was better than they expected."