Wenger managed Sanchez for three-and-a-half seasons at the Emirates Stadium before Sanchez moved to United in January, and the Frenchman gave his take on why the Chilean has not enjoyed the best of starts at Old Trafford:
"It is the question he has raised with his performances. I believe he has lost confidence. The strength of Alexis Sanchez is to take initiative to dribble, to take people on.
"They as well these kinds of players who are most vulnerable when they have no confidence any more, because their game is based on confidence and feeling free to take initiative, he has lost that slowly, even with the interruption of the season.
"Since the start of the season he had a high level of physical energy, but he has lost [that] as well."
Sanchez joined the Gunners from Barcelona and was a crucial player during his time there, contributing 80 goals and 45 assists in 166 appearances.
However, since joining United, he has produced just four goals and seven assists in 28 appearances.
His drop-off in form can be traced back to his time at Arsenal. After plundering 30 goals and 18 assists in his final full season with the Gunners, he contributed just eight and four, respectively, between the start of last season and his departure in January.
The 29-year-old may have been fatigued after four consecutive tournaments with Chile between 2014 and 2017, but after a summer off this year he has not improved.
Football writer Liam Canning and Bleacher Report's Rob Blanchette have seen some encouraging signs from the forward in recent matches, though:
Rob Blanchette @_Rob_B
I think Sanchez set the tone tonight. Was disappointed in Pogba’s general play, but the team looked like a slick unit - something we’ve not been able to say in similar games this term. Tonight, Mourinho got a real performance out of his team. Credit to him and the players. #MUFC
Sanchez is yet to show whether he can still be the player United need him to be. At Arsenal, he was an electric presence in the final third, consistently ruthless and creative in front of goal.
If he can rediscover that—and if his confidence is the issue then United must do whatever they can to improve it—he could have a transformative effect on the Red Devils performances and ambitions.