What you need to know:
- Goals from Mario Pasalic and Merih Demiral had Ole Gunnar Solskjaer's men on course for a fifth defeat in eight games.
- However, United's attacking arsenal blew the Italians away after the break as Marcus Rashford and Harry Maguire drew them level before Ronaldo powered home a header nine minutes from time.
- Ronaldo also scored a 96th-minute winner against Villarreal in United's previous Champions League game to mask an otherwise unconvincing performance.
Manchester, United Kingdom
Manchester United needed another Champions League fightback finished off by Cristiano Ronaldo as they came from two goals down to beat Atalanta 3-2 at Old Trafford on Wednesday.
Goals from Mario Pasalic and Merih Demiral had Ole Gunnar Solskjaer's men on course for a fifth defeat in eight games.
However, United's attacking arsenal blew the Italians away after the break as Marcus Rashford and Harry Maguire drew them level before Ronaldo powered home a header nine minutes from time.
Ronaldo also scored a 96th-minute winner against Villarreal in United's previous Champions League game to mask an otherwise unconvincing performance.
Questions will continue to be asked of Solskjaer's ability to get the best out of such a richly talented squad.
But a much-needed win to move to the top of Group F eases the mounting pressure on the Norwegian.
Solskjaer responded to Saturday's 4-2 defeat at Leicester by dropping Jadon Sancho and Paul Pogba.
Rashford was handed his first start of the season after shoulder surgery and should have scored after just three minutes when he fired wide when clean through on goal, although the offside flag was later raised against the England international.
United's major problems, though, have come at the other end as they have now kept one clean sheet in 20 games.
Atalanta played through the hosts with ease for the opening goal as Josip Ilicic fed Davide Zappacosta down the right and his low cross was turned home by Pasalic.
Fred was denied an instant equaliser by a brilliant save from Juan Musso before United were easily breached again.
Maguire endured a torrid return from injury against his former club at the weekend and was outjumped by Demiral as the Turkish defender headed home from a corner.
Old Trafford was growing restless, but luckily for United they were up against a defence as suspect as their own.
Fred fired another huge chance wide before Rashford hit the bar with just Musso to beat before the break.
Rashford finally found his range to halve the home side's deficit with a calm finish from Bruno Fernandes' pass eight minutes into the second half.
Scott McTominay then fired against the post and Musso was forced to full stretch to turn a low Ronaldo shot to safety.
Solskjaer unloaded his wealth of attacking options off the bench as Pogba, Sancho and Edinson Cavani were introduced for the final quarter.
But United's awful defending nearly gave them too much of a mountain to climb.
Duvan Zapata spun Victor Lindelof with ease and forced David de Gea into a stunning save before the Spaniard parried Ruslan Malinovskyi's follow-up strike over.
It took just two minutes to prove the value of those saves as Maguire hammered home at the back post from Cavani's flick-on.
Fernandes should have completed the comeback when he fired too close to Musso.
But that cleared the way for Ronaldo to provide another fairytale ending as he rose highest to power home Luke Shaw's cross for his sixth goal in nine appearances since returning to the club where he first became a world star.