Fernandes has barely had a break since arriving at Old Trafford nearly a year ago but the Portugal international insisted he is not feeling the effects of the busy schedule.
"Tired? With 26 years old I can't be tired," Fernandes said. "If I'm tired now, when I arrive at 30, 32, I will not play. I will play one game in five [days] so, no, I'm not tired."
Fernandes came off the bench to score the winner in the 3-2 victory over Liverpool in the FA Cup fourth round on Sunday, only the sixth game he has failed to start this season.
"Of course the coach knows what is better for the team and today he chose to play with Donny to give some freshness to the team," Fernandes added.
"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."
Manchester City will have the chance to move to the top of the Premier League table when they travel to West Bromwich Albion on Tuesday but United can reclaim top spot when they take on Sheffield United.
Solskjaer's team are heading into a crucial run with games against Arsenal, Southampton and Everton to come before their next FA Cup tie and Fernandes said beating Liverpool can provide a timely boost.
""I think it's important because you go to the next round, this was the most important," Fernandes said.
"Of course for the confidence of the team, it's really important to keep winning and everyone knows winning against Liverpool is never easy and we make it.
"But now we have to focus on the next game because it'll be hard in the same way it was this one."