The powerful frontman spent last season on loan at Goodison Park and impressed by finishing as their top scorer with 15 Premier League goals.
Lukaku's form persuaded the Toffees to smash their transfer record to land the 21-year-old for a huge £28 million on Wednesday evening.
Following the switch, Mourinho explained that Lukaku was allowed to leave as he lacked the necessary motivation to force his way into the first team at Stamford Bridge.
In a withering parting shot, the Portuguese coach claimed that his former player wanted to be a first choice striker, but he wasn't prepared to work for the spot.
He told reporters: “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 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.”
Lukaku was signed for around £18 million from Anderlecht in 2011, but the striker failed to truly breakthrough at Stamford Bridge.
He spent a successful campaign on loan with West Bromwich Albion in 2012 before last season's move to Everton, and he was forced to deny reports of a rift with Mourinho last year.
It's been a superb couple of days for the Toffees off the pitch - Lukaku's five-year contract came hot on the heels of Ross Barkley signing a new deal which keeps him at the club until 2018.
Everton begin their Premier League campaign with a trip to Leicester City on the opening day, while Chelsea travel to Burnley on Monday August 18.