Chelsea's frontman was found in space behind Mertesacker, who couldn't make contact with the ball when sliding to recover as the last man. Sky Sports Statto captured the defender's reaction to the inevitable red card:
Sky Sports Statto @SkySportsStatto
Per Mertesacker's red card is the 12th in 48 PL games between the sides. It's the German's 2nd red card for Arsenal https://t.co/ivQy4ECYO12016-1-24 16:22:00
Bleacher Report's Chelsea correspondent Garry Hayes felt it was deserved punishment as Costa was denied a goalscoring chance:
Garry Hayes @garryhayes
Mertesacker off here for hacking down Costa who looked to be through on goal. Never a dull moment when #CFC face #AFC. Definite red, too.2016-1-24 16:18:50
In truth, the centre-back's severe lack of pace was brutally exposed by Costa, as noted by Bleacher Report UK:
Bleacher Report UK @br_uk
Per Mertesacker tracking back on defence. https://t.co/7kn6H1pOXK2016-1-24 16:22:19
Bookmakers Coral struck a similar tone:
Mertesacker trying to catch Costa: https://t.co/JzCMBY4DM42016-1-24 16:26:28
It's the second time in as many meetings between these two teams that Arsenal have finished without 11 men. Gabriel Paulista and Santi Cazorla were sent off in their trip to Stamford Bridge last September, where they went on to suffer a 2-0 defeat.
According to KICCA's Ian Stafford, this is one time too many in which Mertesacker's pace—or lack thereof—has let him down, and manager Arsene Wenger should be pushed to sign a replacement:
Ian Stafford @IanStaffs
Think it's clear now. Arsenal need to sign a world-class central defender to join Koscielny. Mertesacker is just too slow. #AFC2016-1-24 16:27:59
On the other side of the argument, Tom Williams of Agence France-Presse suggested Costa made the most of his tumble in attempting to influence referee Mark Clattenburg's decision:
Tom Williams @tomwfootball
The slow-motion replays of Costa's tumble after Mertesacker's foul are tremendous. Almost the full Klinsmann.2016-1-24 16:30:17
A week of the January transfer transfer window remains, but it's unlikely one red card will convince Wenger to change his squad—especially when those plans include demoting his vice-captain.
Brazilian Gabriel will be expected to line up alongside Laurent Koscielny in Mertesacker's absence over the next few games after he came on as a substitute for Olivier Giroud during Sunday's match.