Arsenal manager Arsene Wenger has conceded that Laurent Koscielny is highly unlikely to return against Newcastle United this weekend, which means the Gunners will have to plan for the Premier League showdown with just one fit central defender available.
Koscielny has missed Arsenal's last two matches with tendinitis in both his Achilles tendons, a long-term problem which has blighted the French international's season and must be managed very carefully.
Wenger is already without Calum Chambers through suspension and can't call upon Nacho Monreal to provide cover either, while there's problems going further forward involving Aaron Ramsey.
Who will Arsenal turn to?
Arsenal's lack of cover at centre-back is truly frightening, with even a youngster with one first-team start in Issac Hayden also sidelined. If Koscielny fails to improve over the next 48 hours then utility man Mathieu Flamini might even have to slot in as an emergency centre-back, with your guess as good as mine beyond that.
Youngsters Steffan O'Connor was given a chance against Galatasaray in midweek, although is surely not ready to partner Mertesacker for 90 minutes against a Newcastle side in terrific form.
Big problems for Wenger
During a pre-match injury update on Arsenal's official website Wenger admitted that he's threadbare at the back ahead of a return to domestic action, while Ramsey is also nursing a problem and Theo Walcott is still not ready for a return to first-team contention.
Wenger said: "Our main problems are still defensively because Laurent Koscielny (calf from Southampton match) looks like he will not be quite ready, Nacho Monreal (foot) is not back and Calum Chambers is suspended.
"Mikel Arteta (calf) is not ready and Theo Walcott (groin) is out. Aaron Ramsey (hamstring) will be checked today and tomorrow, so we’ll see."
Wilshere claims rubbished
The Gunners boss also hit out at reports that Jack Wilshere has suffered a setback in his rehabilitation from an ankle operation, rubbishing those claims as simply untrue.
"That’s completely wrong, Wenger said: "He’s doing very well. He’s on crutches so I don’t see how he can have a setback.
"It’s frustrating because it’s hard enough for Jack to be injured. He doesn’t need the negative news on top of that. It’s always frustrating to deny things that are not true.
Taking Wilshere out of the picture there's no question Wenger has massive problems to cope with on the injury front, desperate for positive news on Koscielny's fitness in particular.