Like in the Red Devils' win over Villarreal, Ronaldo got the decisive goal, which was a brilliant header from a cross from the left.
This seemed unthinkable at half time as Man United found themselves 2-0 down thanks to goals from Mario Pasalic and Merih Demiral.
Marcus Rashford, who had earlier hit the crossbar from close range, begun the fightback early in the second half.
There was also redemption for Harry Maguire, who had a shocker at Leicester and didn't cover himself in glory for Atalanta's second, equalised with a well-taken finish.
And the comeback was complete thanks to another marvellous Ronaldo moment, which has put Man United in charge of their Champions League group while also easing some of the pressure on Solskjaer.
The victory provided a much-needed shot in the arm for United, who moved top of the group and welcome rivals Liverpool to Old Trafford in the Premier League on Sunday afternoon.
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