LONDON -- Arsenal manager Arsene Wenger expects centre-back Shkodran Mustafi to be sidelined for at least three weeks with a hamstring strain after getting injured in Saturday's 3-1 win over Stoke.
Mustafi had to come off after just 25 minutes at the Emirates, dealing an injury blow to the Gunners going into the busy Christmas period.
The defender has formed an impressive partnership with Laurent Koscielny since his £35 million summer move from Valencia, but Arsenal will now have to do without him in several crucial league games, including next weekend's visit to Manchester City.
"It's a bad one," Wenger said about the German's hamstring strain. "The minimum [layoff] is 21 days. ... That means he'll have a nice Christmas."
Mustafi was rested for Arsenal's midweek Champions League game at Basel and Wenger confirmed he had been carrying a minor knock ahead of that game, but had been cleared by the medical staff ahead of Saturday.
"He had nothing. I would not have played him today if he had [an injury]," Wenger said. "Even he wanted to play in Basel, because he said, 'No it's nothing,' and he was completely cleared. I checked three times, there was no problem before. I don't know why it happened. But it looks a good hamstring strain."
Mustafi was replaced by the returning Hector Bellerin, with Gabriel Paulista moving into his natural centre-back position after starting the game at right-back. Gabriel has shown good form in recent games, but losing Mustafi still threatens to disrupt what has been a solid defensive unit for Arsenal.
Bellerin set up Arsenal's first goal, sending in a cross for Theo Walcott to level after Charlie Adam had given Stoke the lead from the penalty spot. Mesut Ozil gave the Gunners the lead after the break and Alex Iwobi made sure of the win with his second goal in two games.
It took Arsenal provisionally top of the Premier League table on goals scored, although Chelsea can reclaim the lead on Sunday.
Arsenal have now given up penalties in each of their last three home games, and this one came after Granit Xhaka collided with Joe Allen in the area and seemed to put his elbow up in the Stoke player's face. Wenger, though, was adamant that it was the wrong decision.
"We have now had three penalties at home, not one was convincing to me, and especially the one today. And despite that we came back and won the game," the Arsenal boss said.
Asked why he was not convinced, Wenger added: "Because it was no penalty. It wasn't a foul. There was no foul at all ... I don't think that there was an attempt to make a penalty at all, there was no foul."
The win rounded off a perfect week for Arsenal, who beat West Ham 5-1 in the previous round and then won their Champions League group after a 4-1 victory at Basel.
Wenger paid tribute to his team's ability to keep calm after that setback even though Wenger himself was angrily arguing with the fourth official for several minutes.
"The key was to not get my frustration into the game. And to keep our nerves and our composure. And that's what we did. We kept passing the ball and didn't panic," he said. "That's maybe linked with the fact that we had some positive results for a while now, and that we have created a momentum that gives us strength to believe in our game."
It was another sign that Arsenal's title credentials have strengthened this season, after their challenge fell apart in the spring when they dropped points at home repeatedly against mid-table teams.
"When you analyse last year's season, at home we did really well against the top five. And we dropped points between five and 12 at home, teams that were in mid-table," Wenger said. "Certainly that's our challenge this year, to continue to do well against the top teams but as well in this kind of game, like Stoke today where you have to win, we dropped some points last year."