The Portugal international scored a 78th-minute free kick to send United through to a fifth round tie against West Ham, but only after being rotated out of the starting line-up by manager Ole Gunnar Solskjaer and replaced by Donny van de Beek.
Fernandes had no issue with Solskjaer’s decision, having started 25 of United’s 29 games to date this season, but he denied that his minutes are being managed because he is suffering from fatigue.
“Tired? With 26 years old I can't be tired,” he said. “If I'm tired now, when I arrive at 30, 32, I will not play. I will play one game in five so, no, I'm not tired.
“Of course the coach knows what is better for the team and today he choose to play with Donny to give some freshness to the team.
“I think it's fair enough because Donny is training really well, he's been important for us and will be more and more important in the future.”
Fernandes will celebrate the first anniversary of his arrival in Manchester later this week and Sunday’s goal - his first against Liverpool - will rank as one of the highlights of his first year at United.
The 26-year-old is now focused on bringing his first trophy to Old Trafford having played in three of United’s four semi-final defeats over the past 12 months.
Solskjaer’s side remain top of the Premier League - two points ahead of second-place Manchester City, who have a game in hand - and hope that they remain there after Wednesday night’s visit from Sheffield United.
“I come to the club because I knew I will come to a club who wants to win everything,” Fernandes said.
“My mentality and my way to be in my life and football is about winning, and so you have to be confident, you have to trust your team-mates, you have to trust yourself, and you have to believe,” he said.
"And I will believe until it's not more possible, you know? Like when I see someone get the trophy before me then I will [have] lost my belief.”
Fernandes added: “I always was like this. I don't like to lose, so I think maybe the mentality is more like I don't like to lose. I don't conform with losing games is normal stuff. For me, losing is not normal so my mentality comes from that, you know?
"Also playing whatever it is, when I was playing with my friends, my brother, with my neighbours, I never want to lose so I want to win every time. This mentality will be with me for the rest of my life, I think."