The duo played together at Ligue 2 side Tours during the 2008-09 season, and Giroud is impressed with the centre-back's improvement since then.
"I'd say he's one of the best in the Premier League," Giroud told Arsenal's official website. "I think he can continue to [get even better].
"He's very respectable and serious in his work. You can trust him, and his behaviour on the pitch is important. I like when a guy concentrates and is serious.
"He is like a leader on the pitch, by his behaviour. He is important for the national team now, and he is a very important player for Arsenal. He is a very good player and a very good guy."
Arsenal have picked up five points from three Premier League games so far this season.