Mertesacker has been hugely impressed with his compatriot's contribution, but says he must remain patient.
"He always takes his opportunity to make a great impact and that's what we need," the defender told Arsenal Player. "I'm very pleased for him and for our team that his contribution to us, at his age, is absolutely amazing this season. He came in against Swansea, did a good job and scored the first goal.
"When I see him every day in training you can see that someone is coming but we try to keep him calm and he takes it really seriously.
"He's got a big chance to achieve something and be a regular player at this club so he needs to be patient. Sometimes there will be little setbacks when all of the other players are coming back.
"But I think how he has done and how he reacted to different situations has always been good."
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