When Laurent Koscielny was stretchered off against Manchester City, Arsenal fans could be forgiven for having a sinking feeling. Keeping City at bay without Koscielny would prove to be no easy task, but Gunners supporters will also have recognised that his absence could have implications far beyond the game at the Etihad.
Losing one match to Manchester City is one thing. Losing Koscielny for a prolonged period would be quite another.
The Frenchman's partnership with Per Mertesacker has been the foundation of Arsenal's recent success. The pair have formed a hugely effective duo and had even put together a run of four consecutive clean sheets between November and December.
We don't yet know how long Koscielny will be absent. Arsenal have not commented on the extent of his injury except to say that he suffered a "deep laceration" to his knee. That will provide some comfort to Arsenal fans who initially feared a cruciate ligament injury.
When Koscielny was withdrawn, Thomas Vermaelen entered the fray. Vermaelen is a capable deputy: the Belgian is still Arsenal's club captain and a regular international.
Arsene Wenger will hope his skipper seizes the opportunity that has been handed to him and produces the performances of which he is clearly capable.
However, beyond Vermaelen, there is no real depth to speak of.
Arsenal's other option at centre-back is Bacary Sagna. However, he is comfortably the club's best right-back and is required to remain stationed on the flank.
Having let Johan Djourou and Sebastien Squillaci leave the club this summer, and with youngster Ignasi Miquel currently on loan at Leicester, Arsenal are desperately short of defensive cover. The next in line is teenage prospect Isaac Hayden.
For much of the season, the received wisdom has been that Arsenal needed to sign a centre-forward in January. However, Koscielny's injury serves as a timely reminder that Arsenal also require a centre-half.
What do Arsenal need in January?
Arguably, their need is even greater in defence than up top. At least in the event of an injury to Olivier Giroud, Arsenal are able to call on the likes of Lukas Podolski, Nicklas Bendtner, Theo Walcott and Yaya Sanogo. Arsenal have potential replacements, even if their calibre is questionable.
Should Mertesacker join Koscielny on the sidelines, Arsenal would struggle to find a fourth centre-back without compromising the full-back positions. They simply don't have the numbers to cope with any more injuries.
Of course, finding a player of the requisite quality who is happy to be back-up for Mertesacker and Koscielny is not going to be easy.
The temptation may be to target a player at either end of their career: either a youngster still learning their trade or a more experienced figure who is content not to play every game.
However, Arsenal should not settle for a below-par signing just to pad out the squad. Players thrive on competition. As good as Mertesacker and Koscielny have been this season, they might be even better with the threat of another established defender ready to take their place.
John Cross of The Mirror has suggested St. Etienne's Kurt Zouma could be a target. What Zouma lacks in experience, he makes up for in pedigree: The 19-year-old has already been earmarked as a future France international.
What's certain is that Arsene Wenger will be scouting the continent for potential defensive recruits. Koscielny's injury will have reminded him of how close he is to a potential crisis at the back.
James McNicholas is Bleacher Report's lead Arsenal correspondent and will be following the club from a London base throughout the 2013/14 season. Follow him on Twitter here.