Despite the furore surrounding the sacking of Roberto Di Matteo and subsequent appointment of Rafael Benitez, Mancini said Chelsea could not be ruled out.
"It's difficult to say this because the season is long and I think in February when the Champions League [ Images ] (starts) everything can change," the Italian told reporters on Friday ahead of Saturday's home match with Everton (1500 GMT).
"I think Chelsea could win. In this moment I think (it's a three-horse race) but it's difficult to say."
He is a doubt for the midweek Champions League match with Borussia Dortmund and derby with United next weekend.
However, Mancini was hopeful that Mario Balotelli [ Images ] would kick on from scoring his first Premier League goal of the season against Wigan.
"Now I hope that he will continue to score because we need that, we need his goals," he said.