"We know the quality of the players who have come in, but the team was already very good. We showed it last season. We didn't win trophies, but the team was growing up," Fernandes said after the game.
"Our focus has to be game by game. We'll achieve something with this mentality."
Leeds got back into the game three minutes into the second half with a stunning strike from Ayling.
It took United just three minutes to restore their lead, however, after launching a counter-attack, Pogba found Greenwood who ran down the left wing and nestled the ball into the net.
Fernandes got his second on 54 minutes. It looked like Ayling had cleared the ball off the line but referee Paul Tierney got the signal on his wrist watch that the ball had crossed the line.
The Portugal international completed his hat trick -- his first since joining United -- on the hour mark with a powerful strike past Meslier.
"My objective every season is being better than the last one. Every performance I do, I want to do better and better. I've started with three goals but it's a long way [to go]," Fernandes said.
Pogba secured his fourth assist on 68 minutes when he squared a ball for Fred who hit it into the net from 10 yards out.
"People talk about the quality of Paul, but his quality is not in discussion. We know what he can do and today he showed that," Fernandes said.
"He's an important player for us. We know to expect these kind of performances from Paul."