The German struggled up against Shane Long.
After their win against Manchester City there is no doubt that Arsenal would not have expected what happened in the fixture with Southampton.
The Saints ran rampant at St Mary's, winning 4-0 against the title challengers.
Cuco Martina scored an absolute screamer to open the scoring but then Arsenal's defence was tormented by Shane Long. He bagged two goals with his raw and sometimes sketchy performance whilst Jose Fonte headed in from close range.
Arsenal had their complaints after the game regarding offsides, fouls and what-not for the goals, however they did not help themselves - especially Per Mertesacker.
The German defender has never been quick but he has developed a way of dealing with pacy forwards. Yesterday however was not one of those performances.
Long had him, and in fairness Laurent Koscielny, on toast for much of the game with his raw pace, strength and use of the dark arts of football doing the trick.
Here is how the Arsenal fans reacted to the performance...
You would think Per Mertesacker, a man standing at 6ft6in, could handle a shoulder barge, but referee books Shane Long— Simon Peach (@SimonPeach) December 26, 2015
Missing person. Per Mertesacker lost Shane Long at about 1945 tonight and has been trying to find him since. Please help. #Where'sShane— Denzil (@DenzilsBiscuits) December 26, 2015
Koscielny and Mertesacker always look suspect playing against forwards that are mobile & threaten behind. Shane Long been excellence tonight— Callum (@CoachCallum86) December 26, 2015
When you look at Mertesacker struggling against the pace of Shane Long, you can only hope he loses his passport before the away leg in Barca— IronMan (@IronMan_AV) December 26, 2015
What a terrible performance! Whole team was so poor,Mertesacker is so fragile