It was the first game since January 2023 that Mount stayed on for the full 90 minutes, with his last completed fixture coming in a Chelsea shirt, also against Palace.
With forwards Matheus Cunha and Benjamin Sesko out of the squad due to injuries, Mount was named in the starting XI at Selhurst Park, and his goal in the 63rd minute from a set-piece sealed United's 2-1 win.
"Mason is a great player," Fernandes told BBC's Match of the Day. "His quality was never in doubt with his team-mates.
"Sometimes he struggled a bit for goals and assists and also with injuries, but when he gets his fitness, he is important."
Mount, who signed with United in July 2023, missed 50 games across competitions due to injuries in his first two seasons with the club. He suffered a knock in August this year, causing him to sit out two more games. But the English midfielder said he is feeling good after playing an entire match.
"The win was massive but also, for me, to play the full game. I haven't done that in a while," the 26-year-old told TNT.
"I have been feeling really good recently, enjoying my football. When you have days like this in front of the fans it makes it all worth it."
United's victory, only their second on the road in the last 12 games, took them to seventh in the league table with 21 points, nine behind leaders Arsenal. They will next host West Ham on Thursday.