United won the first-leg of their Europa League semifinal 6-2, having at one stage been 2-1 behind.
Fernandes told Sky Italia: "We didn't get anxious, we were in the game with our minds.
"We didn't expect Roma to be so aggressive, we were good at finding the players between the lines, playing the ball quickly on the ground. Then Roma were unlucky, it had to make three changes in the first half without the possibility of making others in the second half.
"(Amadou) Diawara made an enormous effort, he could no longer run. Roma paid for the exhaustion, we were good at creating more chances and scoring goals."