Mourinho, who gave his first press conference as United boss on Tuesday, first made his name in English football at Stamford Bridge, winning two Premier League titles during a glittering first spell at the club.
The Portuguese tactician made a sensational return to the Blues in mid-2013, but despite earning another Premier League title during his second season back, Mourinho was dismissed in December of last term after losing nine of 16 league games.
And Sir Alex believes that returning to Chelsea was a mistake for the former Real Madrid and Inter Milan boss.
He told Sky Sports at a recent charity event: "I thought Jose was wrong to go back to Chelsea, I really do.
"I think it's always difficult going back to somewhere for a second time."
However, the Scot, who won 38 trophies during his tenure at Old Trafford, believes that Mourinho's difficulties last season will not harm his legacy in the game, adding he was confident that his former club had made the right appointment.
"The ability, charisma and personality, his success is high, won a European Cup twice with different clubs, won a Europa [League] with Porto, winning titles with all the teams he's managed. His record - you can't ignore that.
"I think Manchester United is the right kind of club for him, I really do. I think we'll do well."