The Blues have moved quickly in the summer transfer market, bringing in Cesc Fabregas before the World Cup, and have since added fellow Spaniard Diego Costa and also defender Filipe Luis from Atletico Madrid, as well as promising 19-year-old midfielder Mario Pasalic from Hajduk Split.
Mourinho, however, understands all too well each of the other leading sides have also strengthened their armoury, while Manchester United are expected to recover and challenge again under new manager Luis van Gaal.
"Last year, if you had asked me in July, I would never have said Man United would be outside the top four. This season I will tell you one (to miss out) and I have a big percentage of making a mistake," said Mourinho, who is an ambassador and pundit for BT Sport next season and took part in a Q&A session at BT Tower in central London on Friday night.
"There were good teams last season and there will be better teams this season because everyone is investing.
"A few years ago you make a team to be champions and most probably you are champions. Now if you make a team to be champions, you try to finish in the top four."
Mourinho added: "Why I like the Premier League so much is that at this moment, nobody knows who is going to win the title.
"I know that we want to and I know that we can, but it is the kind of competition where I can't say.
"At this moment, I think between us, Liverpool, Arsenal, Man United, Man City - who is going to be first, who is going to play in the Europa League, I don't know."
Mourinho maintains it took him only 20 minutes to convince Fabregas to sign from Barcelona - and reckons the former Arsenal captain never wanted to move back to his old club.
Arsenal had first option to bring Fabregas back to the club he left in 2007, which manager Arsene Wenger passed up as he pursued other targets.
Mourinho, though, feels Stamford Bridge was always the midfielder's preferred destination.
"I spoke with him 20 minutes. I think he really wanted to come to us," he said.
"As you know, Arsenal had an option where they could interfere, but I think he was not open to that, I think he was very, very much in our direction, so it was an easy job for me."
Mourinho indicated the Blues' chequebook was now closed.
"I have to say my club did a fantastic job, and not just because of what we bought, but because we did it in almost record time. The transfer market closes 31 August and we close our market on July 19. We finished the market today," he said.
"The club did fantastic, we knew the targets, we knew the players and the club attacked them and their clubs at a very early stage.
"We got exactly what we need and what we want. The squad is one which I like very much and I look forward to starting the season."
Mourinho added: "I don't think so (sign any more players). The market is open, but we are so happy with what we have."
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- Cesc Fabregas
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