Heinze joined Arsenal boss Mikel Arteta's backroom staff at the start of this season and has made a big contribution with the Gunners top of the Premier League table.
Fernandez coached both Heinze and Arteta at PSG in 2001 and he told BBC Sport: "Gabi lives for football.
"He's always studying the game, and especially the work of the defensive players. These days you have a goalkeeping coach, a set-piece coach, it's important to have someone coaching the defenders too.
"You can always improve. Work on your positioning, tackling, distribution. That's where Gabi can help. When you have somebody of his experience telling you something, you listen. I'm certain he'll be improving those defenders."
Fernandez added, "The strength of a manager lies in his ability to surround himself with people he trusts completely.
"Mikel has made a great choice. Gabi is loyal. He's straight-talking, a real, sincere person and Mikel knows he'll support him every single day."