Onana was superb on the night, which marked a first game in charge for new manager Ruben Amorim.
He said: "Well, not so good, especially because of the result.
"We are Manchester United and it’s a lot of responsibility on us. We have to be able to win games. Unfortunately, we didn’t win today, so it’s difficult.
"I think we have to continue working hard. Me, the first one, I will take responsibility. Of course, I’m happy with these saves, but the most important thing is the victory of the team.
"When you don’t win, it’s difficult, so you have to move on, take responsibility and be positive."
Onana also said: "We are such a big club so there is no time to cry, we have to move on.
"Good days will come, better days will come. Like I always say, it will not rain forever here and the sun will shine.
"We know it’s just a matter of time. We have to continue working hard and pushing ourselves, stay together and stay positive, because in the end there are still a lot of games to go, a lot of competitions where we can fight.
"So we are all excited and very positive about the season."