At his best, the German is a positionally astute centre-back at ease on the ball. But his loss of form is far from the only thing ailing Arsenal defensively

At times this season Arsenal’s Premier League campaign has resembled a rust-ridden Victorian water system. Plug a leak here and another drip starts bubbling out over there. Shift the pressure away from one dribbling fissure: there it goes, spurting out at the other end.

The hole where there really ought to be a pair of high-class central midfielders has been effectively bunged at times, not so at others. The absence of Mesut Özil, Aaron Ramsey and Jack Wilshere has been compensated for by excellence elsewhere, most notably Santi Cazorla’s fine recent run of form.

The pressure will always out though and the past two weeks have seen a leak in an unexpected place. Per Mertesacker has been steady for Arsenal in the last two seasons: not an eye-catching central defender, but a quiet leader and a rare point of consistency. How much more alarming, then, for Arsène Wenger that the most vitriolic criticism currently should centre on Mertesacker’s meekness at vital moments, an embarrassing – and instantly punished – inability to challenge for the high ball, even accusations of outright cowardice in failing to defend crosses more bullishly.

Twice in two weeks the German has been vilified by the combined punditry might of Jamie Carragher and Martin Keown. Just as twice Mertesacker has turned his back on the ball in the lead-up to an opposition goal, most damagingly as Martin Skrtel leapt to power in Liverpool’s stirring injury‑time equaliser at Anfield. Certainly, the evidence on Sunday evening was fairly clear: give a retired English centre-half a light pen, a three-minute punditry slot on Match of the Day 2 and a cowering German centre-half and this is only ever going to go one way.

It is a painful, if slightly obvious point of comparison. At Anfield Skrtel had just had the back of his head stapled together. Keown himself was relentlessly tenacious as a player, as was Carragher, who was equally critical of Mertesacker’s inability to lead a callow defence against Stoke. Elsewhere on Sunday afternoon Steven Taylor had just prevented Sunderland from scoring by hurling his face at the post. Look, though, at the cowardly German! Look at him ducking and squirming. As exercises in punditry go this is, let’s face it, a bit like shooting a tall, polite, slightly immobile fish in a barrel.

And yet it would perhaps be better to understand a little more and condemn a little less.

Keown, Carragher, Skrtel, Taylor, Mertesacker: one of these players has over 100 international caps and has played at three World Cups. Clue: it is the one who now appears to be afraid of challenging for the ball. The suspicion is that there might just be another explanation for some abject defending beyond simple faintheartedness.

Some have already blamed a disastrous execution of zonal marking. Against Liverpool Mertesacker seemed to be passing the need to challenge for the ball on to another player, a case of the system falling to pieces as players lose their bearings or forget the primary job is still to attack the ball. It looked terrible. But in shrinking back presumably Mertesacker imagined he was clearing the way for a colleague to attack the cross rather than simply realising, 12 years into an elite level professional career, that actually he’d rather concede a goal than have to put up with any physical contact.

This is perhaps part of the issue here. Mertesacker is a confusing player, a defender who suffers from what we might call Crouch’s Paradox. Despite being 6ft 6in tall he isn’t particularly good at heading the ball unless it falls directly out of the sky on to the top of his head. As with Crouch, the problem is a lack of speed and lateral mobility, just as heading the ball well in open play is a function of spring and explosive power as much as inches (Andy Carroll, a majestic header of the ball, manages to combine both).

Mertesacker has never been mobile. He is instead a player of craft and quiet intelligence – a brain on stilts.

As a young man his nicknames in the Bundesliga were The Defence Pole (for his slenderness) and Mr Clean (for his lack of bookings), neither of which suggest the kind of boxing-glove-on-a-spring central defender English football has traditionally felt more comfortable with.

Beyond this Mertesacker’s best attribute – his positional sense – responds to a settled partnership. There is no doubt Arsenal have missed Laurent Koscielny. This season they have lost only once in the league (to Chelsea) with Koscielny in the team and kept clean sheets in 42% of the games he’s played, as opposed to 23% without him. Mertesacker has been most obviously affected. Of the two it is the Frenchman who attacks the ball, on the floor and in the air.

Mertesacker meanwhile brings other qualities: positional sense, strength in the more static clinches, and reliable distribution. In the 2012-13 his pass completion rate of 91.5% was the fourth highest in England’s top flight. This season he is up around the 90% mark again. Mertesacker may not have stitches in his head, or a Keown-like ability to balk and block and smother but he is undeniably brave on the ball, where a more wholehearted English-style centre-half will so often resort to the panic-stricken punt into the channels. There is no doubt which kind of bravery – physical or footballing – Wenger prizes most highly.

Yet this is of course a defence that can only go so far. Mertesacker is undoubtedly playing below his best. Which is where the real issue here – lop-sided player recruitment – raises its head once again. The fact is with a single injury, to Koscielny, Wenger has lost the ability to rest or rotate his other senior centre-half. There is nowhere for Mertesacker to go right now.

And just as the real reason Arsenal were defending so deep against 10 men at Anfield was the inability of the midfield to control the tempo or pass Liverpool out of the game, so the humbling of Mertesacker is a case of referred pain, a usually reliable part being stretched to show its weaknesses by failings elsewhere. Mertesacker has never really been a backs-to-the-wall man for the defensive trenches. He is also vulnerable to periods of poor form now and then. The fact that he is being asked to do the former, while having no respite from the latter is a function of a failure to reinforce, to upgrade and to give a struggling stand-in captain somewhere to hide.