After a lukewarm first period, Ronaldo scored six minutes into the second half.
The Portuguese picked up Scott McTominay’s pass before skirting to the right of the Brighton defense and, barely backing away, sent a low shot past a static Robert Sanchez.
Three minutes later, Brighton captain Lewis Dunk was sent off after VAR sent referee Peter Bankes to review the defender’s foul on Anthony Elanga.
Fernandes squandered a glorious opportunity when he was unable to get past Sánchez with only the Brighton keeper to beat, before Ronaldo headed an excellent chance just wide of the far post.
That meant United breathed a sigh of relief when Jakub Moder’s speculative effort from the corner of the box bounced off the bar with David de Gea beaten.
Brighton was still trying to salvage some of the competition when Fernandes practically hit with the final kick.