Sanchez, since joining the Red Devils has suffered a torrid spell, with injuries having most part of his career at the Old Trafford.
Speaking on BT Sport, Van Persie believes the former Barcelona winger, who enjoyed happy moments sometime ago at the club is now unhappy.
"Sanchez doesn’t look happy to me.
"I can’t properly judge it because I am not working with him but when I see him play I rarely see him smiling.
"Of course, I get it: when you are playing you need to be serious and you want to win.
"But after you score a goal you can smile, because that is the point of football.
"We all play football because we like it and if you like something you should smile.
"I think in the first couple of months after his move he was happy and he was playing well.
'Mentally he needs to find that happiness again because if you are happy then you are fitter, you want to create and you want to have fun.
"I don’t know if it’s the pressure. Pressure is something you can’t see or touch but he has to manage that in his head.
"He is still the same great player. He has it all: he’s fast, he can assist, he can dribble, and he can score.
"There is just a lot of negativity around him from fans and people who have opinions on social media.
"Maybe it’s all become a little too much for him.
"He just needs to block it out, find his true happiness and just enjoy football."
