In a pre-season friendly in the United States, Manchester United took the lead with a long-range strike from their new captain.
It is the first game for the Portugal international since being given the armband by Erik ten Hag.
But it was an effort that Ramsdale should have saved as Fernandes' effort crashed in from outside the box.
The Arsenal goalkeeper got a hand to it but it was not strong enough as it bounced through and into the net, giving United the lead at the MetLife Stadium.
And he did not cover himself in further glory when Jadon Sancho raced through to make it 2-0 shortly after.
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Gabriel made a hash of a clearance, kicking the air after a ball over the top found the England international.
One-on-one with Ramsdale from a tight angle, Sancho lashed it into the roof of the net but the Gunners goalkeeper was easily beaten at his near post, and the scored ended 2-0.
Fernandes was named as United captain this week after Harry Maguire was stripped of the armband.
The England international was left furious with the decision by Ten Hag, who has largely left the defender out of the starting line-up since taking over last summer.
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However, the Dutchman might feel vindicated in his decision after the Red Devils finished last season with the most clean sheets in the Premier League.
Fernandes adopted the armband last season and was United captain in all but name, though now he has since been formally recognised as the skipper.
And he showed his leadership qualities at half-time when he was hilariously seen pulling his manager away from berating a match official.
It is just further evidence that Ten Hag is not scared to make the big decisions, though it now leaves Maguire's future uncertain.
As for new arrivals, only Mason Mount started for the Red Devils from the summer signings.
Andre Onana has joined from Inter Milan but was not in the squad for Saturday night's game against Arsenal.
As for the Gunners, Kai Havertz was making his second start after scoring his first goal for his new club in a friendly against the MLS All-Stars.
Jurrien Timber was also named in the starting XI, as well as Declan Rice, who joined from West Ham for £105million.