The South American forward moved to Manchester United in a swap deal from Arsenal in the January transfer window.
Sanchez is the best paid player at Old Trafford after the Red Devils handed the Chile international a contract worth an estimated £350,000 a week.
The 29-year-old scored three times in 18 games in all competitions in the second half of the 2017-18 campaign.
Sanchez has failed to get on the score sheet in the current campaign to raise questions about Manchester United’s decision to sign the former FC Barcelona star.
The Chile international lasted 63 minutes of Manchester United’s 1-1 draw with Wolverhampton Wanderers at Old Trafford in the Premier League at the weekend.
Former Arsenal star Merson believes Sanchez doesn’t suit Jose Mourinho’s desired style of play at Manchester United.
Merson told Sky Sports: “At United, there’s a structure and they say ‘let’s not get beat first and then go from there’.
“That’s why I think he’s finding it difficult. At Arsenal, it was free-flowing but it wasn’t successful, they didn’t win anything.”
He added: “It’s the pressure. When he was at Arsenal, he was the top man. Everything went through him, plus he could try things at Arsenal. He could flick a ball here, flick a ball there and if it didn’t come off, you were playing next week. That is not the case at Old Trafford.
“With Jose Mourinho, we saw what he was at Chelsea with Eden Hazard, if you are not getting back and following the full-back back, you’re off and you’re not playing next week. That didn’t happen at Arsenal.”
Manchester United are in seventh place in the Premier League table after the Red Devils dropped points against Wolves.
The 20-time English champions signed Shakhtar Donetsk midfielder Fred, FC Porto defender Diogo Dalot and Stoke City goalkeeper Lee Grant.
The Red Devils lost to Chelsea FC in the FA Cup final last term to end Mourinho’s second season without silverware.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news