The Gunners moved back to the top of the table as they came from behind to win 3-1 at West Ham United.
"We need the right additions. We said that a lot in the summer then Mesut Ozil came, but at this time, in a very special period that only happens in England, you need to get the right additions with players who come back from injury," Mertesacker told Arsenal Player.
"We lost Aaron Ramsey [against West Ham] so a good player is maybe out for two or three weeks, so we need the right additions now.
"We have a good squad and that is going to be vital in the next couple of weeks."