The Portuguese signed Eto'o from Russian side Anzhi Makhachkala in August, with the two having working alongside one another previously at Inter.
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The relationship between them in Milan was reportedly fractious, and while the Cameroon international insists that is now much improved, he has revealed that he had once made up his mind that he would never play for Mourinho.
"Before meeting up at Inter, Jose and I didn't know each other personally, so our relationship was a little tense," he told The Sun.
"I think I even said one time that I’d never play for a club where Jose was coach.
"But God knows best. He wanted to show me that I was wrong and today Jose is a friend. Now he’s my coach again."
Chelsea have dropped just two points in their opening three games of the Premier League season prior to Eto'o's arrival at the club.
Mourinho's men face Everton on Saturday, and Eto'o claims that he has enjoyed his time working with the Portuguese again so far.
"It's a real pleasure to team up with him [Mourinho] again," he continued.
"He's as good on the pitch as outside of it, and this is a real and rare quality for a manager."