Arsenal striker Gabriel Jesus was left out of the team for their game against Liverpool due to injury.
The forward played against Nottingham Forest in midweek and was training with the team.
But manager Mikel Arteta confirmed that Jesus, who has been playing with fluid in his knee, suffered a setback.
Arteta told reporters: "Unfortunately, he had a setback in training and he wasn't fit to play.
It's something related to his knee, but I think it's a short-term thing."
Arteta added: " The return [against Forest] might've been a bit too fast for him."