The 50-year-old embarks on his second stint at Stamford Bridge after leaving the club by mutual consent in September 2007.
Mourinho believes Manchester United, Manchester City, Chelsea, Arsenal, Tottenham and Liverpool are all contenders for the Premier League title next term, and claims the English top flight is the most competitive division in Europe.
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"In my time there were three teams. Everybody knew that between us, [Manchester] United and Arsenal, one is first, one is second and one is third.
"At this moment, [Manchester] City have appeared with fantastic economical power which has helped them go up as fast as they did.
"Tottenham had a very good period with Harry [Redknapp], reaching fourth and playing Champions League, going up and up and up.
"Andre [Villas-Boas] did a good job by keeping them at that level and they have conditions to fight for the Premier League, not just for a Champions League spot.
"Liverpool, I know Brendan [Rodgers], I know he can do it. Now, I don't speak with him so much on the phone, but last year I did, and I know his project, ideas and what he wants to do, so there are about six teams.
"Who's going to be first? Who's going to be sixth? I don't know, but for sure, of the more important leagues, it is the one with the most contenders."