The Chile international moved to Manchester United in a swap deal from Arsenal in the January transfer window.
Sanchez opted to move to Old Trafford despite being heavily linked with move to Premier League leaders Manchester City over the past 12 months.
The 29-year-old struck an unhappy figure at Arsenal over the past 12 months but his January move hasn’t triggered a drastic improvement in form.
In fact, Sanchez has only managed to score one goal in 11 appearances in all competitions since his move to Manchester United.
Reports in the British media last week suggested that Sanchez has already found himself ostracised at Old Trafford following his poor performances.
Former American goalkeeper Keller believes Sanchez should retire if he is still unhappy at Manchester United.
“It’s hard to have any empathy for someone as successful as he is and the kind of money he’s making,” Keller told ESPN FC.
“You work hard, that’s part of your job – you’re a footballer.
“It’s part of the fans expecting from you a level of performance and professionalism.
“You can always retire. You don’t have to get paid what you’re getting.”
Manchester United are in second place in the Premier League table and two points ahead of Liverpool FC in the race to finish as runners up behind Manchester City.
The Red Devils lost 2-1 to La Liga outfit Sevilla in the Champions League round of 16 earlier this month.
Manchester United finished in sixth spot in the Premier League last term.
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