The defender has struggled since signing from Chelsea, making high-profile errors in Premier League games against Liverpool and north London rival Tottenham.
Arsenal is fifth into the table ahead of Monday's (AEST) trip to Watford and Gunners academy manager Mertesacker feels the Brazilian will bounce back.
"I think he will learn from his mistakes on the pitch, but he is very comfortable being at Arsenal," Mertesacker said.
"That's what I have been seeing. So we are happy to have him.
"He brings something different to the table right now. So I think he is able to learn and excel his level of being an Arsenal player."
Arsenal began a match with their starry front three for the first time against Tottenham as Unai Emery selected new signing Nicolas Pepe alongside Alexandre Lacazette and Pierre-Emerick Aubameyang in attack.
Lacazette and Aubameyang scored to help Arsenal fight back from 2-0 down to claim a draw at home, but Mertesacker feels the Gunners may need a tactical rethink.
"I think we have a lack of balance right now, where you feel like, OK we have amazing strikers but they as well need to understand that we need to have a certain compactness," Mertesacker said.
"It's not only about our three strikers providing an offensive threat. It is about these front three as well, leading the first front line as being a defensive unit.
"So I think we need to be cautious of that. No matter how much striking power you have got, you need to have a sense of defensive compactness from everyone."