This is according to former defender of France and Chelsea Marcel Desailly.
Manchester United won 10 of his 12 games from interim boss Ole Gunnar Solskjaer since Mourinho was fired in December.
The only defeat of Solskayer's reign came in the first Champions League match at the last 16th Old Trafford PSG meeting on Tuesday.
Goals from Presnel Kimpembe and Kylian Mbappe provide a 2-0 aggregate lead for League One.
And after the game Desailly says there was a known problem in Manchester United's defense.
Jose Mourinho wanted to improve his defense at the transfer market last summer, but failed to deliver one of his goals.
"What was the problem with Mourinho still there, he is still there," Deainil told BEIN SPORT.
"They have 10 amazing games, but they're still there.
– You talk about defense, the lack of leaders, it's still there.
"Today she appeared in the Champions League.
"Solskjaer and players must get their motivation to prove they are a great club.
"We have determined what is necessary.
"It's the same for Chelsea when they've lost [Manchester] You are.
"You know you've discovered that you have a problem in your back where you need to fix it.
"This is what top-level game gives you as an indication."
United return to action on Monday against Chelsea in the fifth round of the FA Cup.