The 27-year-old scored twice for his country as they saw off Uruguay 3-1 in a World Cup qualifier, despite carrying a hamstring injury going into the game.
Wenger warned ahead of the game that it could be a "suicidal decision" for Sanchez to play given he was struggling with the problem he had picked up earlier in the international break.
But the 27-year-old started and played 84 minutes and will not return to Arsenal's London Colney training base until Friday.
With the Gunners kicking off at Old Trafford in Saturday's early kick-off, Sanchez faces a race against time to prove his fitness - with Wenger keen to observe the former Barcelona man in action for Chile before making the call.
"I have watched the goals and he looked very sharp," he said.
"I will watch the game
I know him so well, like all my players that I observe every day, so I will be able to see if he was playing with a restriction or not.
"But the first goal he scored he ran from the halfway line and resists - if you are really injured with a hamstring you cannot do that."
Sanchez has a history for wanting to play through any pain barriers as Wenger found out to his cost almost 12 months ago.
After scoring a brace in a Champions League win over Dynamo Zagreb, the Frenchman confirmed Sanchez had a hamstring problem - but he still opted to play him in the next Premier League outing at Norwich.
There, he lasted an hour before being forced off and he would miss almost two months as Arsenal fell out of the title picture.
Wenger has said he will ask the player for an honest assessment of the injury as he looks to pick up his first-ever Premier League win over Jose Mourinho and move to the top of the league table.
Asked if Sanchez's self-diagnosis can be trusted, Wenger replied: "He is always okay! It is a very difficult one because I am sure he will come back saying he is all right."
Asked about Sanchez's fitness, he added: " I haven't heard a lot because we were in doubt 24 hours before the game against Uruguay
"He texted us to tell us he's in good shape after the game
He came off after 84 minutes and now we have to wait
I see him tomorrow morning and then I will see if he can be involved in the squad or not.
"What I will consider is the risk of injury because he played while recovering from a hamstring injury
I'll consider the way he feels as well
On that front you depend on the honesty of the player, how they feel and how they recover."
Two players who definitely will not be involved at Old Trafford are Spain internationals Santi Cazorla and Hector Bellerin.
Cazorla is still recovering from an Achilles problem while Bellerin faces a month on the sidelines after picking up an ankle injury in the win over Tottenham.
"It was the last 10 seconds, you watch it again," Wenger said of the tackle from Danny Rose which caused the issue.
"It was a bad one
It's annoying to lose Bellerin, that's for sure
Did Rose want to injure Bellerin? I don't think so
He just went in for the tackle."
Bellerin could now be out until the game at Manchester City in December but, having been linked with a move to Pep Guardiola's team, Wenger believes the highly-rated 21-year-old is close to agreeing terms on a new deal at the Emirates Stadium.
"We are working on that," he said.
"And we are not far
It will be great
Let's say 'it is great' when it is done."
With Arsenal able to go top, and United looking for a win to keep in touch with the top four, the game itself is set to be an intriguing affair.
Wenger is keen for all interest to remain on the pitch, rather than ramping up the relationship between himself and Mourinho - a man he has never beaten in the Premier League.
"You know, we didn't lose always - you present that," he said of his record against the ex-Chelsea boss.
"We have beaten them and there was many draws as well
I think I have won against every manager in the world during my 20 years here and I do not make of this game a competition between two managers.
"It's between two clubs and two teams and I think I can understand that people want to create controversy, but it's not that that makes the audience."
Source : PA
Source: PA