The Chile international was thought to be on the brink of a move to Manchester City in the January transfer window.
However, Manchester United pulled off a transfer masterstroke after seeing off competition from their derby rivals to sign Sanchez.
The 29-year-old, who is the best paid player at Old Trafford, scored three goals in 18 games in all competitions in the current campaign.
Sanchez has failed to find the net in Manchester United’s opening five Premier League games to raise questions about the South American.
The former Arsenal striker produced an underwhelming performance in Manchester United’s 1-1 draw with Wolverhampton Wanderers at Old Trafford last weekend.
Ex-Gunners star Merson believes Sanchez looks lost at Manchester United since his move to Old Trafford.
“He looks a bit lost at the moment, but that could be confidence,” Merson told Sky Sports. “I can’t believe it’s nearly six months without a goal. For someone who is supposed to be that good, that’s a long time as a forward.
“He’s struggling at Manchester United. They don’t make as many chances as Arsenal. Arsenal are all-out attack but that’s not the way they play at Old Trafford. When Arsenal play, they just go gung-ho, that’s not the case at Manchester United.”
Manchester United finished in second spot in the Premier League table last season to secure their best finish since the Sir Alex Ferguson era in 2013.
The Red Devils haven’t won the Premier League title in the past five seasons and Manchester United are already eight points off the pace this term.
Sanchez won two FA Cup titles during his three and a half seasons at Arsenal.
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