The Europa League setback was Xhaka's first appearance since losing the captaincy after shouting abuse at the home support.
After the 2-1 defeat, Keown remarked: "I thought perhaps Xhaka should've gone off at half-time. He didn't look really right.
"I can understand why he stayed on, I think he wanted to show that bit of heart, that determination.
"I do give him credit for that, but this is when a manager has to really know, is a player fit or not?
"I think the medial ligament there really opens up, and the player knows. After that he limped around. He didn't want to go off.
"It's not really intelligent for the team. I don't want to blame him for this, but I was really surprised to see him come out for the second half.
"I've had this injury myself, it's really tricky. It's hard to question it. Maybe Emery wasn't really sure what he was looking at.
"But (Kamada's second) goal, he just can't move. Maybe that's his mobility, but I think there's an injury there with the player.
"I don't think he was completely honest.
"He stayed on the pitch and, in the end, it became a disaster for the manager, because how many players are really fit here?"