Although the game ended 4-3 in the Bundesliga side's favour, the Cameroon international feels it was his failure to keep out Leroy Sane's shot, after the Reds had started so well and looked comfortable, was a crucial moment in the tie.
Bayern took control to extend their unbeaten run in the group stages to 35 games, a competition record, stretching back six years.
It may have been an unhappy European debut for the club for Onana but he did make a number of good saves in the second half, particularly one outstanding stop to tip away an Eric Choupo-Moting drive.