Arsenal have been rocked by the news that Shkodran Mustafi has been injured playing for Germany during their final qualifier for the World Cup against Azerbaijan.
The German international seemingly pulled up with a hamstring injury during the match and has definitely been ruled out of Arsenal's away trip to Watford this Saturday. The extent of the injury is not known quite yet, but their are fears he could be out for more two months.
With Laurent Koscielny still a doubt, Arsenal find themselves short of options at the back in their three-man defence.
Provided Koscielny returns to fitness, here are four solutions manager Arsene Wenger could use this weekend.
1. Bring in Per Mertesacker
The most logical replacement for the injured Shkodran Mustafi is club captain Per Mertesacker.
The former German international was a first team regular for Arsenal over a year ago when playing alongside Laurent Koscielny, but a long term injury in pre-season last year meant that he missed all but two games last season.
Since the start of this season, Mertesacker has yet to feature in the Premier League for the Gunners, being restricted to a handful of appearances in cup competitions. However, now is the perfect opportunity for the World Cup winner to get back in the first team in the league.
His good performances so far this season in the cups, his superb display against Chelsea in last season's FA Cup final victory and his many years of experience mean he should be a more than competent replacement for Mustafi.
2. Keep the Faith With Rob Holding
The young Arsenal man enjoyed a great start with the Gunners last season after joining them from Bolton, even finishing the campaign by lifting the FA Cup. But a shaky start this season against Leicester City and Liverpool meant that the Englishman was dropped for Premier League matches, with Monreal and Mustafi being preferred to him.
Since the Liverpool game in August, Holding has played mainly in cup competitions in the Europa League and league cup, but returned in the Premier League during Arsenal's last game against Brighton in the 2-0 victory.
With Mustafi's injury, Arsene Wenger could look to keep Holding in the side, and if he does, It would be a good chance for the 22-year-old to remind Arsenal what he is capable of after his below par start this season.
3. Revert to 4 at the Back
Another good option for the Gunners could be to bring back the four-man defence they used for many years under Arsene Wenger.
If they are unsure about relying on young Holding and Monreal in the three-man defence for the upcoming match, a traditional four-man defence could be reliable way to cope with the loss of Mustafi. Arsenal should be more than comfortable with playing in that system after using it for many years.
It could mean that Arsene Wenger may reinstate the Mertesacker-Koscielny partnership of seasons before or play Holding with Koscielny instead. Either way it could give the Gunners a bit more flexibility in the centre of their defence by playing two centre defenders at the back, given their options without Mustafi.
4. The Elneny experiment
One final (more extreme) option Arsene Wenger could try is bringing midfielder Mohamed Elneny in the back three.
Wenger tried this during Arsenal's pre-season games ealier this year. Speaking about Elneny playing as a centre back to Arsenal's official website, Wenger said: "Elneny is a player who has a fantastic mentality and attitude, and his main target is to help the team. When he plays there, of course he does extremely well."
Although the Arsenal manager appears to show faith in Elneny's ability to play in a back three, in remains to be seen whether he would try this in a competitive match. But this is another option if Arsene Wenger was serious about his comments towards the Egyptian midfielder.