The Chilean joined Inter Milan on loan this summer after a season and a half at Old Trafford plagued by a lack of fitness and form.
Mourinho was manager for the first year of Sanchez’s time at United, and while suggesting some degree of culpability in his favour, believes that the former Arsenal forward was unhappy at the club.
“Sanchez… I felt him [to be] not a happy man,” Mourinho told The Telegraph.
“And I think in every job you have when you are not happy it is not so easy to perform at every level. And maybe I am wrong.
“Maybe it was me who was not capable to get into him and to get the best out of him.”
Sanchez scored just three Premier League goals for Manchester United before being loaned to Italy this summer.
Inter Milan have also signed Romelu Lukaku from United, another marquee signing who did not produce their best at Manchester United.
And Mourinho hopes Sanchez will rediscover his happiness and form alongside Lukaku in Milan.
“As a manager sometimes you have the capacity to get the best out of the players and other times you are not successful in that approach.
“But the reality is that I always felt [of Sanchez], ‘a sad man’.
“So probably in Italy he will recover this. I hope he can. I always wish well to every player.”