However, both Sanchez and German World Cup winner Mesut Ozil could depart in January to avoid them leaving as free agents in the summer.
"Half in half out, you never know this kind of situation during the transfer period," Wenger told the BBC after Sunday's disappointing defeat.
"That is why I decided to focus on the game (and leave him out)."
Before the game -- which Arsenal lost despite taking the lead early in the second half -- Wenger said that a decision on Sanchez's future was "imminent".
"Yes (it's imminent). Don't read too much in it because even I don't know which way it will go."
"He's being vague at the moment. The situation is not completely decided one way or the other, so I left him at home."
One of the Premier League's brightest attacking players, Sanchez has cut an increasingly forlorn figure during a disappointing campaign.
Sixth-placed Arsenal are five points behind Tottenham Hotspur in fifth and, after defeat on the south coast, face an uphill task if they are to avoid a second successive season without Champions League football.
"We were 1-0 up and suddenly we lost two goals and we don't know where they came from," Wenger added.
"It is very frustrating. Overall we have to look at it in a very objective and harsh way we made mistakes we should not have made. We had to win the game."
In four seasons at the Emirates Stadium, Sanchez has won two FA Cups.
(Reporting By Tom Hayward; Editing by Toby Davis)
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