Some fans turned on Ferguson after the Manchester derby defeat at Old Trafford on Tuesday over his part in appointing David Moyes.
But that prompted Mourinho, a close friend of Fergie, to speak out in his defence.
The Chelsea manager said: "He made United what United is. He deserves to retire and enjoy life. 'Now he's responsible for what? For what? United lose a match and it's his responsibility?
"United struggle in the Premier League and it's his problem? (Wayne) Rooney misses a goal, or (David) De Gea makes a mistake and it's his fault? Let him retire in peace."
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