The Arsenal defender is a throwback to centre-halves of old and he's not known to milk injuries unless he's seriously hurt.
And there was no doubt about the damage left on the former German international following a collision with Gary Cahill in the first half of Sunday's Community Shield.
Competing for the ball in the air, Cahill rose highest and saw his left elbow make contact with Mertesacker's eyebrow.
And, all of a sudden, there was blood all over the Wembley surface.
https://www.youtube.com/watch?v=OqGRiGUSYjY
Mertesacker hit the deck instantly and BT Sport cameras captured the red geyser as it poured from the 32-year-old's forehead.
With stitches certainly required, the cut was so bad that manager Arsene Wenger was forced to withdraw Mertesacker and replace him with new signing Sead Kolasinac.
Mertesacker is certainly going to feel that one tomorrow.
