Alexis Sanchez has struggled at Manchester United because he is not happy, according to former star Robin van Persie.
Sanchez has failed to live up to the hype since swapping Arsenal for Premier League rival United in January 2018, scoring just five goals in 45 appearances.
Injuries have also hampered Chile international Sanchez, who managed only one league goal for the Red Devils last season.
Van Persie also made the move from Arsenal to United in 2012-13, winning the Premier League in his first season at Old Trafford – the club has not topped the English summit since.
Now retired, Van Persie tried to explain why Sanchez has failed to make an impact under Jose Mourinho and Ole Gunnar Solskjaer in Manchester.
"He doesn't look very happy to me," Van Persie said via the Daily Mail. "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."
"It's a different time," Van Persie told the Telegraph. "Over the past couple of years a lot has changed. There are a lot of impressions from fans, from social media, he is constantly getting hit about his wages. It's negative, so he starts negative, and maybe that is difficult."
Sanchez was lured to United by Mourinho last year and the Portuguese boss was sacked in December amid poor results and criticism over his perceived brand of defensive football.
Van Persie added: "For him it was difficult because he was coming into a defensive playing team. I came into a team with all the mature players there already: Rio Ferdinand, Nemanja Vidic, Paul Scholes, Ryan Giggs, Michael Carrick. At Arsenal I was one of the older ones, captain, and here I could just come in and have fun, because the guys sorted all the other stuff beside it.
"Mentally, I was just having fun. I was happy, I was just enjoying it, really. I did not think about the consequences. I was not using social media back then, so I was not really bothered about what people said. When I went outside it was all positive so I was in my own little bubble of being happy."
Solskjaer was installed as interim manager in December before the United great signed a permanent deal in March as the club eventually finished sixth and 32 points off the pace.
"When you play for a club like Manchester United, the fans are used to attacking football and risks," Van Persie continued. "I don't really think the fans were disappointed about not winning the league. You can miss out on winning the league because you have great teams in this league. It's maybe the way they played that the fans were not really happy with.
"United is like a beast. In a positive way, when you are doing well. It is so huge. It is fantastic, a great place to be. But when it goes wrong, when you're having a bad period as a player or a coach, it's tough. It's a tough place, lots of pressure, everyone demanding not only a win, but they are demanding attacking football. 77,000 people want to be enjoying it."
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