The New York Red Bulls striker is expected to sign for the Gunners on-loan in the coming days.
"I am very happy to train with him. He is training very well," Mertesacker told arsenal.com.
"He has a lot of experience and he is a great opportunity for us, but I am not the manager and I do not know what is possible, for him or for us. In training he shows his class and [it is good] to have him with us."