Sir Alex Ferguson claims Jose Mourinho's decision to return to Chelsea as manager in 2013 was a mistake.
The Portuguese, who was this week officially unveiled as Manchester United's new manager, quit Real Madrid three years ago in favour of starting a second spell in charge of the Blues.
After a trophyless first season back at Stamford Bridge, he guided the club to the Premier League title the following campaign, but Chelsea imploded spectacularly last term and he was sacked in December with the club flirting with the relegation zone.
And Ferguson, who has previously claimed Mourinho would've been a candidate to directly replace him at United had he not already agreed to return to Chelsea, says he should never have gone back in the first place.
"I think Jose was wrong to go back to Chelsea. It's always difficult when you go back somewhere for a second time," Mourinho told Sky Sports.
Ferguson does, however, think Mourinho will be a success at United after three disappointing years since his own departure.
"I think he'll be a massive improvement, I really do," the Scot added.
"In terms of ability, charisma, personality, success he's had, he's won the Champions League with two different clubs, he won league titles with all the teams he's been at. His record, you can't ignore.
"I think United is the right kind of club for him and I think he'll do really well."