"I knew him from my days at Chelsea," Gallas told Arsenal.com. "He is a good player, very strong and with lots of quality. He is a very strong player and he wants to play.
"I was not really surprised [when he signed for Arsenal]. I knew he would come and train hard. Some were surprised, but I was not because he is a good player."
With their relationship already strengthened through the French national team, Gallas was able to play a key role in helping Diarra settle into life at Emirates Stadium.
"I just told him when he arrived to be patient," he said. "He really wanted to show everybody what he can do and I told him to be patient and your chance will come.
"I talk to him all the time to try and point him in the right away and I think everything will be okay."