Aaron Wan-Bissaka extended Manchester United's lead in the first minute of added time before Fernandes teed up Marcus Rashford to secure a resounding 4-1 win.
Five United stars who started the loss to Tottenham were replaced for the game against Newcastle United.
David de Gea had to be equally sharp to claw away a stabbed Callum Wilson effort off the line at the other end, and it was not until four minutes from time that United slipped into top gear.
Maguire and his teammates came in for plenty of criticism following their below-par performance in the 6-1 home thrashing by Tottenham Hotspur two weeks ago but the 27-year-old dismissed talk of a crisis.
Newcastle fans are also still waiting to hear whether their season ticket costs will be refunded as a result of not being able to attend games. We've won 4-1 today and you're asking why I didn't start one of the players. "There is a long way to go, we have to improve", he told Sky Sports. He gave me a great pass and I just needed to do the rest'. Some games we will not be at our best but we have to do the basics well and in the games we lost we didn't do the basics well.
"Attacking-wise, I thought Dan with the speed would create problems for teams and Juan and Bruno are intelligent and create chances for us and we could have scored a few more".
"Everyone is talking about we need a defender, midfielder, a striker, a winger, it's easier talking from outside saying we need everything without being on the pitch. He showed good character, I am happy for him", said the Norwegian.
Manchester United now take a trip to Paris to kick off their Champions League campaign against PSG in midweek, before they host Chelsea in the Premier League next weekend.