The Portuguese heaped praise on the Belgian forward, who has scored 15 goals in 32 games for the Toffees, and also hailed Thibaut Courtois' time at Atletico Madrid
The Blues forward looked set to push for a breakthrough into the first team this season before heading for Goodison Park late in the summer transfer window, where he has scored 15 goals in 32 games across all competitions.
With Atletico Madrid striker Diego Costa on his way to Stamford Bridge in the summer, Lukaku's future is uncertain - with the Toffees and Tottenham both interested in a permanent move - but the Portuguese recognises that he has benefited from his time away.
"Romelu Lukaku had what I call a successful loan," Mourinho told Chelsea's official website.
"Sometimes players go on loan and, at the end of the season, we cannot find one positive reason for that loan.
"But Lukaku played all the time - apart from when he was injured - so he has minutes of experience in the Premier League in a good team, with a good coach and scoring his goals.
"You can say the same thing about Thibaut Courtois [at Atletico]. A successful case of loans are the ones we have to analyse.
"My final report [on the season, given to the club on Monday] will have in detail these kinds of cases.
"That's my job - to analyse things, have an opinion and make my decisions.
"I have to work with the club and respect the club's decisions but I know clearly what I would like for next season."
The Blues boss also praised Eden Hazard, with whom he has disagreed on tactics in public of late but who was a standout player for both Chelsea and the Premier League in the opening months of the season.
Mourinho observed: "Eden has had a big evolution in his game and a big evolution in his personality. He had a high that lasted for a long time - many, many months.
"He couldn't finish on a high because he was injured and in the most important moment of the season - for the second leg against Paris, for the first leg against Atletico - he was not there.
"Against Atletico in the second leg he was there but not in top condition. He wasn't there for the last few matches of the Premier League because of his injury but he was clearly a player with a big evolution and hopefully he will be even better next season."