The injury picked up by Shkodran Mustafi on international duty will leave the German sidelined for up to six weeks, according to Arsene Wenger.
In the short-term that presents the Arsenal manager with a problem, because the 25-year-old has been a regular in the side for the last few weeks and the Gunners have some other injury worries at centre-back. In the long-term, it brings into sharp contrast the age profiles of his central defenders and what a job he's got on his hands over the coming months.
Of the six recognised central defenders at the club -- and Nacho Monreal is included because although he's spent most of his career at left-back he's a fixture in that position these days -- three of them are over 30.
Per Mertesacker, the club captain, is 33 and retiring at the end of the season. Monreal will turn 32 in February, and while hardly over the hill is at an age where you have to start to consider how you're going to replace him. And Laurent Koscielny is also 32, but the injury problem which might keep him out of Saturday's trip to Watford is one which will trouble him for the rest of his career.
The French international has a chronic Achilles' tendon issue, something which requires daily treatment, and while manageable for now could well contribute to him calling time on his playing days earlier than he might like.
On the other end of the spectrum, both Rob Holding and Calum Chambers are 22, with the latter having been awarded a new contract this week.
"I believe that it's important we keep our young centre-backs because they blossom at 24," said Wenger on Thursday, but perhaps more than any other position on the pitch, experience in central defence is crucial. Young players with lots of promise like Chambers and Holding can only achieve their full potential through education.
Some of that comes from playing games and learning from mistakes, but the advice and guidance of senior professionals plays a big part in that too. When you have someone who has been there and done that to advise you what to do at key moments, it's hugely beneficial.Mustafi injury with Germany 171008 Getty Images
The evidence of that was clear in the final part of last season when Holding played in almost every game up to and including the FA Cup final. He generally had Koscielny or Monreal in there with him, and at Wembley in May the veteran Mertesacker was there to help him cope with the guile of someone like Diego Costa, a player who lives on the edge and has made life uncomfortable for even the best defenders in the Premier League and Europe.
At the moment the blend of relative youth and experience is quite good, but if they lose too much of the latter too quickly, then it could start to become problematic. During Wenger's time at the club young defenders have burst on to the scene, and showed great early promise.
Perhaps there are no better examples of this than the Swiss duo of Philippe Senderos and Johan Djourou. Such was the potential of the former than he was very seriously considered for the captaincy at a very early stage of his Arsenal career.
However, over time they were left exposed by Arsenal's defensive frailties, and while the weaknesses in their own games may have become apparent anyway, it's also fair to say that they didn't have the right kind of experience alongside them to guide them through those difficult periods.
As it stands, the trio of Mertesacker, Monreal and Koscielny are all intelligent players and intelligent men, perfect teachers for the likes of Chambers, Holding and even Mustafi who, despite being 25, has somewhat youthful flaws in his game that really need to be ironed out.
Soon enough, the baton will need to be passed and the gaps filled. It's harder than ever to find top class central defenders these days, even if you have plenty of money to spend, but while Arsenal fans fret over how they might replace the potential departures of stars like Mesut Ozil and Alexis Sanchez, Wenger has to ensure he makes a good case for the defence as well.