The former United striker says Alexis needs to find a way to be happier in his work.
"He doesn't look very happy to me," Van Persie says. "He looked happy at Arsenal. He doesn't look happy now. What I've seen and felt over the years is that how you feel mentally, how your life is, has a big influence."
He adds: "If you're happy, you're 50 per cent fitter, you create more and you enjoy yourself.... so he should find a way mentally to change that.
"Of course, pressure is there because you are playing for a big club and everyone has opinions about you. But you have to manage that."