Usually mild-mannered, Mertesacker looked more like Martin Keown confronting Ruud van Nistelrooy at Old Trafford, waving his arms wildly in the reprimand as the Ox gave the ball away.
The England ace later said: "We set each other high standards and, if you do something that's not good enough, you can bet one of your team-mates will tell you.
"I gave the ball away and it resulted in them hitting the post. I knew I did wrong there. Per just gave me a nice, friendly reminder and that's what you need.
"We do have characters now, strong characters with a lot of experience like Mathieu Flamini and Per, who have been around the game for a long time at a very high standard. And with new signings coming in that makes it even more exciting."
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