That is the opinion of Sky Sports pundit and former Arsenal striker Charlie Nicholas.
Mourinho became Manchester United manager in the summer of 2016 and won three pieces of silverware in his first campaign in charge.
The Portuguese led the Red Devils to second place in the Premier League in his second campaign at the helm behind runaway winners Manchester City.
But this season has been a struggle so far, with United losing three of their first eight Premier League games and sitting eighth in the table.
Jose Mourinho is the favourite with the bookies to be the next Premier League manager to lose his job.
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Nicholas says the 55-year-old has never looked happy during his time at Old Trafford.
And he claims it is unlike Mourinho not to sort out his defence after having such a good defensive record in the past at clubs such as Chelsea.
“It’s like he is falling into this third-season syndrome again,” Nicholas said.
“It just seems like he’s been so unhappy ever since he set foot in Manchester.
“It looks like there’s just been nothing enjoyable for him which is really weird for a guy like him who’s been such a success at all these big clubs.
“The one thing he’d always do is fix the defence and it’s the one thing he’s not managed to do.
“Even his buys are usually good in defence, but he’s really struggled to find the answer at Old Trafford to that defence.
“That’s a big difference as going forward has never been his strength but you could rely on him to fix a defence and this time he hasn’t.”
United return to Premier League action this Saturday when they visit Mourinho’s old side Chelsea.
Maurizio Sarri’s Blues are unbeaten in the league so far and are level on points with City and Liverpool at the top of the table.