Jose Mourinho sold Romelu Lukaku to Everton because he did not believe the striker had the motivation to fight for a place at Chelsea.
Everton completed the club-record signing of the Belgian on Wednesday night, paying a fee of £28million for the 21-year-old who scored 16 goals in 33 appearances on loan at Goodison Park last season.
Lukaku, who has signed a five-year deal, played just 15 times in three years at Stamford Bridge and had not enjoyed the best of relationships with Mourinho after disagreements about his loan spell last summer and then where his future lay in the long term.
But while Lukaku insisted he had no hard feelings towards his former club - despite not speaking to Mourinho prior to his exit - the Chelsea boss made plain why he had seen fit to sell a striker who had once been seen as the heir apparent to Didier Drogba when he signed from Anderlecht in 2011.
"Romelu was always clear that in his mentality and his approach he was not highly motivated to come to a competitive situation at Chelsea," Mourinho said in the Daily Mirror.
"He wanted to play for Chelsea, but clearly only as 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 Financial Fair Play rules, you have to analyse these situations."
Lukaku said he had left Chelsea without speaking to Mourinho.
"I spoke with my agent and then from that moment on I left it up to him as I was on holiday," he said about the move to Everton.
"I made my decision and then didn't speak too much about it."
Asked whether he had spoken to Mourinho at all immediately before his departure he added: "No - I think he was busy with the game (against Vitesse Arnhem on Wednesday night).
"Chelsea is a big club and sometimes it is difficult for young players to come through. I want to have a good career and I didn't want to spend 10 years on the bench, I wanted to make the best out of every year."