Bryan Oviedo’s 86th-minute strike secured the win for Roberto Martinez’s men which has left David Moyes’ side 12 points behind league leaders Arsenal with 14 games played.
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“It’s still early,” said the Frenchman.
“It’s a surprising defeat but it’s still early. We’ve seen it before - 12 points on us - but of course we just have to keep winning to have a chance and not worry too much about the others.”
Chelsea remain second in the table, four points behind Arsenal, following their 4-3 victory over Sunderland at the Stadium of Light.
And when asked if United remain in contention to retain their title, Mourinho replied: “It's difficult but with Manchester United everything is possible.”
Moyes, whose side have won just three out of their seven league games at home this term, concedes United are in need of consistency if they are to mount a challenge in the Premier League.
"We will need to play some great football and win an awful lot of games in the run-in if we are going to be in there and around it come the end of the season," said the Scot.