Torreira was named among the substitutes for the first leg of the Europa League semi final.
Matteo Guendouzi was preferred to the Uruguayan in midfield alongside Granit Xhaka.
Torreira has become a favourite with Arsenal supporters since arriving from Sampdoria last summer.
Before the Valencia match Keown claimed it was a mistake to leave out the all-action 23-year-old.
Maybe it’s fatigue, maybe he feels Torreira has not quite got the legs at the moment
However, he reckoned fatigue was the issue preventing Torreira from playing for the entirety of the match.
“It is [a big night for Guendouzi],” Keown said on BT Sport.
“It’s a surprise and a big night for Guendouzi.
“You have to back the manager, we backed [Arsene] Wenger.
Keown reckons Lucas Torreira was left out of the Arsenal side against Valencia because of fatigue
“For me I think it’s a mistake. I’m not really sure [why the decision was made].
“Maybe it’s fatigue, maybe he feels Torreira has not quite got the legs at the moment, he wants him to play a part later in the match.
“I actually feel [Guendouzi’s] quite a character. He’s only very young.
“It’s whether or not he actually has the experience, whether he’s actually technically good enough.
“Whether he’s quick enough, whether he can sense danger as well.
“All of these elements are really important in the Arsenal midfield.
“Torreira has sort of given Arsenal that tenacity and energy.
“This young man had a fantastic opportunity tonight to do that.
“Can he do it? The question marks are there.”
Torreira replaced Guendouzi after 58 minutes of action at the Emirates.