Diogo Dalot opened the scoring late in the first half before Marcus Rashford powered home a header settle any nerves. Ronaldo wrapped up the win, confirming United’s place in the knockout stages in the process.
Ronaldo was left out of the squad for the weekend Premier League trip to Chelsea for disciplinary reasons and his relief at finally scoring was evident.
United are in second place in Group E with 12 points from five matches ahead of their visit to Real Sociedad in their final pool game next week, where victory could see them top the table and book a place straight into the last 16.
Sociedad defeated Omonia 2-0 thanks to goals from Robert Navarro and Brais Mendez, while Jose Mourinho's AS Roma kept their knockout round hopes alive with a 2-1 win at HJK Helsinki in Group C after striker Tammy Abraham scored their opener.
Additional reporting by Press Association.