Lukaku completed a £28million transfer to Everton on Wednesday, having impressed during a loan spell at Goodison Park last season.
Some observers have questioned the wisdom of Mourinho allowing the 21-year-old Belgium forward, who also enjoyed a productive loan stint at West Brom in the 2012-13 campaign, to move on.
However, the Portuguese has claimed Lukaku's determination to be guaranteed first-team football played a major role in Chelsea accepting Everton's offer, while also acknowledging the financial benefits for the London club.
"Romelu was always very clear with us with his mentality and his approach was not highly motivated to come to a competitive situation with Chelsea," said Mourinho.
"He wanted to play for Chelsea but he clearly wanted to be first-choice striker, but for a club of our dimension it is very difficult to promise to a player. That reduced, immediately, his desire to come to us.
"After that Everton came in with an important offer. As I always say, because it's the reality, our club wants to do things really well. We want to always be inside of the rules. Financial Fair Play is something that is always in the thoughts of Chelsea's board.
"When you have the chance to make some movements that create a positive impact in Financial Fair play, Chelsea always analyse this situation.
"This season, in this transfer window, the situation of David Luiz [who moved to Paris Saint-Germain for a reported fee of £50m] and Romelu leave Chelsea in a great position, even knowing that we also bought a few players."