The Chile international has struggled badly to find consistent form for the Red Devils since his transfer to Old Trafford from Arsenal back in January 2018.
Sanchez struggled with form and fitness issues last season under Jose Mourinho and Ole Gunnar Solskjaer and only scored one goal and made three assists in 20 Premier League appearances.
The 30-year-old has now reported back to Carrington for pre-season training as he bids to regain match fitness after his involvement in the Copa America with Chile this summer.
Now, former Manchester United and Arsenal striker Van Persie feels that Sanchez needs to find happiness away from the pitch before he can be expected to deliver on it.
Speaking in an interview quoted by Metro, Van Persie said: “He doesn’t look very happy to me.
“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.
“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.”
Manchester United are currently preparing for their Premier League season opener against Chelsea FC at Old Trafford on Sunday afternoon.
The Red Devils finished in sixth place in the Premier League and without a trophy last term and they will compete in the Europa League this season as a result.
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