Petr Cech will miss Saturday's huge match with north London rivals Tottenham Hotspur after being injured in the final seconds of the defeat to Swansea City.
Trailing 2-1, Cech raced forward to join team-mates in the box for an attacking corner - but instead hurt his hamstring.
Arsene Wenger confirmed the goalkeeper will miss the game against Spurs on a miserable night at the Emirates that also saw Laurent Koscielny pick up an injury.
"Petr Cech is injured. I think it will keep him out of the game against Tottenham at the weekend. But I have full-confidence in David Ospina," Wenger said after the loss.
Arsenal's third defeat in a row, and second in four days, means they have now won just two of their last eight matches in the Premier League. and four in 12 in all competitions.
"We created many chances and were unlucky in our finishing. It was a very disappointing result," said Wenger after the game.
"I'm worried about results. It's a very difficult moment for the team so we need to focus on ourselves and do the basics.
"Cech got a muscular problem at the end of the game. He will not play on Saturday. I have full faith in Ospina."
"We hit the bar a few times, we were on top, we were unlucky. To lose a game like that is a shame but we have to respond.
"I still can't understand that because it's just under the eyes of the referee and we were unlucky with that decision.
"At the moment we lack a little bit confidence in front of goal. We have to deal with that."
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