Reigning champions City cruised to a resounding 5-0 victory in their opening game against West Ham on Saturday, with Raheem Sterling plundering a hat-trick.
Sergio Aguero surprisingly started on the substitutes' bench, where he was joined by Bernardo Silva, Ilkay Gundogan, Phil Foden, Nicolas Otamendi, Claudio Bravo and Joao Cancelo, City's new signing from Juventus.
Speaking to Sky Sports ahead of Manchester United's clash against Chelsea on Sunday, Mourinho highlighted the pool of talent available to Pep Guardiola.
Asked how many teams can win the title this term, Mourinho replied: "Four. Manchester City, Tottenham, Liverpool and City's B team.
"When I looked at the bench yesterday [Saturday] and the players that were not even involved, I believe even their B team could fight for the title."
"The dressing room must feel they can win the title, if they don't they're in trouble. I think to finish top 4 would be a good achievement for them."— Sky Sports Premier League (@SkySportsPL) August 11, 2019
Jose Mourinho on what would be a successful season for Manchester United this year.
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Mourinho was dismissed by Manchester United in December after a torrid run of form and the Portuguese does not expect his former side to challenge City, Liverpool or Tottenham.
"I want to say the obvious thing: we would love United, Arsenal and Chelsea to be at that level," he added.
"Sometimes in football unbelievable things can happen, but we are speaking about a team that last season won everything, we are speaking about the European champions, we are speaking about the European runners-up.
"We are speaking about United that finished sixth and Arsenal that finished fifth."