Chelsea, who play Crystal Palace on Saturday, are in contention to succeed United as Premier League champions in a title fight with Manchester City and Liverpool.
United are 18 points adrift in seventh place entering this weekend's fixtures as David Moyes, the man Ferguson hand-picked as his successor, endures a challenging first season in charge.
Mourinho, who this term has repeatedly rebuffed suggestions he was a potential successor to Ferguson, said: "Lots of people are speaking about Sir Alex. He made Man United what Man United are.
"He decided to retire. When somebody decides to retire, especially a person like him, he deserves to retire. He deserves to enjoy his life.
"And now he's responsible for what? Man United lose a match it's his responsibility? Man United struggling in the Premier League is his problem?
"(Wayne) Rooney misses a goal, it's his fault? Or (David) de Gea makes a mistake and it's his fault?
"He can only be blamed for one thing: Man United being the club Man United are. He's retired. Let him retire in peace."
Premier League chief executive Richard Scudamore this week suggested an under-performing Manchester United was bad for the competition.
Mourinho, who returned to Chelsea after a near-six-year absence last summer, contends there are other damaging aspects for the Premier League as he once again railed at the performance of referee Chris Foy in the controversial defeat at Aston Villa.
"There are lots of things that are bad for the image of the Premier League," Mourinho said.
"A performance like Mr Foy is not good for the Premier League. Players cheating is not good for the image of the Premier League.
"We have only two teams in the quarter-finals of the Champions League. It's bad that we have no teams in the Europa League."
Like Chelsea, United are in the last eight of the Champions League.
If Moyes wins the European Cup with United, then the success of the season will be reassessed.
Mourinho added: "Why are we speaking about failure, failure, failure? Let him finish the season. If he wins the Champions League..."
It has been suggested Moyes is struggling to motivate his players.
Talking about Chelsea, Mourinho believes a manager can provide additional motivation.
He said: "They (players) need self-motivation, but they also need the extra motivation coming from external sources.
"And the external source, for me, is the manager, is the club, is the fans."
Mourinho may need to call upon his powers at Selhurst Park, a venue few of his players know to play in front of a vociferous home support.
Mourinho said: "We played there in 2004. Many of my players haven't. Some of them even don't know where the stadium is.
"They are noisy, they are supportive of their team. I miss Portsmouth, so I'm happy to go to Crystal Palace."