The French defender has announced his retirement after suffering a serious knee injury in training this week.
Fernandes took to social media to pay tribute to former Barcelona stopper Mathieu.
He wrote: "Today I had news that made me sad and downcast, I knew that the best centre-half I ever had as a teammate was injured and so he decided to end a career full of titles and unforgettable moments! The old man as everyone knew him in the Sportinguista had the misfortune to get injured and thus not end his career as he so deserved.
"I had two and a half years with you, two and a half years of apprenticeship, two and a half years of eternal training bets to see who scored the most free kicks, two and a half years to see magic come from the feet of a central defender, yes from a central defender. The one who normally just defends in the position, but he was more than that. He was a central with quality of a No10 and speed of a winger!
"Thank you for the tears in the final of the Portuguese Cup that showed me what it is like to have love for the game and that all the achievements you counted as if it were the first. Enjoy your family, the golf games and above all to recover! An old hug."