The Gunners suffered a 3-1 defeat against Liverpool on Saturday, with Wenger leaving Sanchez on the substitutes' bench for the opening 45 minutes before introducing the Chile international in the second half.
By that point, Arsenal were trailing by two goals as Liverpool surged into the lead at Anfield and, despite Wenger's explanation that he wanted to play more directly, Keown was highly critical of the call.
"Stunning. I really can't understand why Wenger wouldn't want to play him. He's their best player," the former Arsenal defender said, speaking on BBC's 'Match of the Day'.
"They didn't actually do that [play more direct] in the match. I'm struggling to remember a more tepid Arsenal performance in the last 20 years under Wenger.
"There was a lack of spirit, of fire in their bellies. They just didn't turn up in that first half and left themselves with far too much to do."