The 24-year-old was handed his fifth appearance in the European competition this season but did not do enough to help his side avoid a loss.
The Cameroon international made a terrible blunder in the encounter to allow the Premier League club clinch a place in the next round of the tournament.
Onana allowed a cross from Reds defender Neco Williams to reach Curtis Jones who scored the only goal of the game.
The goalkeeper has admitted his mistake but revealed it could happen to even the best goalkeeper in the world.
“I am not the best, mistakes can happen to anyone. It is difficult for me but I have to take it, it is part of life and it is part of football that is what we train for,” Onana told the club website.
“Sometimes you make mistakes, sometimes not. I am totally responsible for the loss of today. I have to accept the reality, because of me we didn’t draw and we didn’t win.
“I am here to make saves not to make mistakes and I have to be very critical of myself. I have done a lot of good things in the past so I have to push harder.
“I have to keep my head up and then we will see what is going to happen. We played good. We had so many chances. I like the way the players were fighting. We had more possession.
“We had so many chances but we were unlucky we didn’t score today and that is football.”
The defeat saw Ajax drop to third on the Champions League Group D table with seven points from five games.
Erik ten Hag’s men could, however, still secure a place in the knockout stages of the tournament if they beat Atalanta in their last group game at Johan Cruyff Arena on December 9.
“I don’t have to make the mistake again and if we score a lot of goals, I think we will have an opportunity to go through,” he continued.
Onana has made 15 appearances for Ajax across all competitions this season, helping them to keep six clean sheets.