The pair, who arrived last year, had also taken some of the pressure off Paul Pogba, allowing the French midfielder to play to his full potential, he added.
Pogba scored the winner in United’s 2-1 win over Fulham on Wednesday after Cavani had equalised, a victory that took them to the top of the table.
“The last few years, United haven’t been ready to challenge for the title, but now they’ve brought in players like Fernandes and Cavani – players with a winning mentality – that has helped massively,” Derby County manager Rooney said.
“Now the squad is challenging for a Premier League title. I said to the coaches in the office here six weeks ago I thought United will win the League. I still feel that today.
“Paul Pogba will have a massive part to play in that. Those players that have been brought in with winning mentalities will relieve some of the pressure from Paul a bit, and he can now show his quality on the pitch.”
United are two points clear of Manchester City, who have a game in hand. They take on Liverpool in the fourth round of the FA Cup on Sunday before meeting Sheffield United in the league next Wednesday.