"Of course I love it when one of my players becomes a special player in the match," Mourinho told reporters after Chelsea's 3-1 win left the London side just two points behind leaders Arsenal and United 14 points adrift of the top spot.
"It's a fantastic moment for Samuel. He scored a hat-trick and against Manchester United."
Eto'o boosted his overall goal tally to eight for the season so far with two strikes in the first half and a third early in the second, showing the kind of lethal finishing that United sorely lacked despite creating a string of chances.
It was the first hat-trick from the 32-year-old since he moved to England last year from Russia's Anzhi Makhachkala where he was the world's highest paid player, according to media reports.
Mourinho, who has previously expressed frustration at the shortage of goals from Chelsea's strikers, said Spain international Fernando Torres faces "weeks" on the sidelines after hurting a knee ligament shortly after replacing Eto'o as a late substitute.
The Portuguese coach, who recorded his 100th Premier League win, said he selected Eto'o over Torres, who scored in both of Chelsea's previous two league games, because he felt the Cameroonian was more suited to the kind of tight game that United would play.
"I thought Samuel is a player with more technical ability, better in small spaces," Mourinho said.
"It was a bit of a difficult decision. (Fernando) is scoring goals. But for this game I thought that Samuel was more adapted."
(Editing by Toby Davis)
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