The first was from a speculative shot that was going five yards wide and the intervention signalled his unease.
The second in the 97th minute - has assured him of hero status among United followers.
Jordan Larsson, the son of Henrik who begged his father to play with Cristiano Ronaldo and Wayne Rooney back in 2006, had one shot to secure a point for Copenhagen from the penalty spot.
Steve McClaren, the United coach, rushed to the edge of Copenhagen's technical area.
McClaren appeared to shout at Onana: "Don't jump." Onana dived right. His strong hand diverted the ball away from harm's way. Then he heard the final shrill amid the celebratory din.
Onana was mobbed by team-mates and those on the bench rushed to him.
For much of the evening, he was unrecognisable from the confident 'keeper who dictated Inter Milan's play in the Champions League final.
The significance of the save cannot be magnified until the last knockings of the group stage but one point from nine would have been tantamount to a death knell for United. Instead, they are off the mark with a good win that became great with one save.
Something was rotten against the team from Denmark, so United turned to their own Dane to freshen things up.
Erik ten Hag is doubting his own decision-making but also atoning for it.
Christian Eriksen was summoned at the interval to replace the defensive midfielder for the second match running at Old Trafford and galvanised United. They have now won their last three games.
An interrogative Danish journalist had demanded to know why Eriksen was not starting at Ten Hag's pre-match press conference on Tuesday - some United supporters will be posing the same question.
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