The former Defensa y Justicia striker, who spent 2017 on loan with Sarmiento, will spend the 2017/18 season in Ligue 1.
He follows fellow Argentine Geronimo Poblete as coach Philippe Hinschberger restructures with a number of new faces.
He will compete with Nolan Roux and Ibrahima Niane for a place in the club’s attacking line-up.
Fernandez is in line to feature in the league tie with SM Caen ahead of the international break.
The club revealed the deal with a post on their official Twitter account: “It’s official, the Argentinian striker, Brian Fernández, is loaned for a season with option to buy.”