They sit six points behind leaders Manchester City in the Premier League and face champions Leicester City at Old Trafford on Saturday.
When asked about Claudio Ranieri's side and their prospects, Mourinho took the opportunity to take a swipe at his detractors.
"It is very difficult to retain the title," Mourinho told reporters on Friday. "History says that. Not many in the Premier League have done it.
"One team was Manchester United and another was managed by the worst manager in the history of football (Mourinho at Chelsea 2005-2006)."
United captain Wayne Rooney has faced fierce criticism after a string of poor performances in recent weeks but Mourinho said the forward should not be singled out from the rest of the squad.
"Rooney's form is like Man United's form. We are a team - we are not Wayne Rooney, we are a team," the 53-year-old coach said.
"But the way we think in the group is everybody is involved in every match, even the ones who are not playing."
Mourinho, who recently lashed out at football's "Einsteins" who have criticised United during their recent dip, said he was frustrated that he could not protect his players from media scrutiny.
"The only thing that upsets me a little bit is the kind of criticism for my players. I would love to protect my players, but from you (the media) I can't," Mourinho said.
"It is completely out of my control. That gives me a feeling that it is hard. That is frustrating. The Einsteins need money to live... They can speak, they can write.
"They can criticise the work of other people. But I am a good man. I am a man of goodwill. I do lots of charity work. I am helping so many people. Why not also help feed the Einsteins."
Forward Anthony Martial, who was forced off in last weekend's defeat by Watford, will not feature against Leicester due to a calf injury.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by Toby Davis)
