The Belgium international was unveiled as a Toffees player on Wednesday, Everton having agreed to pay a club-record fee to acquire the 21-year-old, who scored 15 goals during his original loan stay at Goodison Park in 2013-14.
And Mourinho has explained his reasons for allowing the prodigious youngster to leave, claiming he lacked the drive to become the club's first-choice striker.
“The thinking was, first of all, the fact that Romelu was always very clear with us that in his mentality and his approach he was not highly motivated to come to a competitive situation at Chelsea,” he said.
“He wanted to play for Chelsea but clearly only as the first choice striker - and at a club of our dimension it’s very difficult to promise a player that status.
“That reduced, immediately, his desire to come to us. After that Everton came with an important offer and because we want to be inside the Financial Fair Play rules, you have to analyse these situations.”
Chelsea spent £32m to sign Diego Costa from Atletico Madrid earlier this summer and also lured fan favourite Didier Drogba back to the club, thus pushing Lukaku down the pecking order.
And the striker has no regrets over his decision to leave the Blues, claiming Everton is the “best place to grow and develop as a footballer.”
“Roberto has been very influential in the deal,” he added at his unveiling. “He is someone who knows football and I want to learn from him and develop under him. He challenged me personally.
“I told him very quickly that I wanted to play under him. The rest was set up by the board and my agent. Now that I’m here I want to help my teammates.
“I decided very quickly that I wanted to come back here. I am 21, I need to play at a high level and with a good team. I needed to be in a place that felt right.
“I have the trust of the supporters here. I can grow and develop. I came back because I have a very good relationship with the players and the staff. I feel like I belong. I haven’t seen any of the lads but I have been in touch with all of them over the last two months.”