The Seagulls won 2-1 with Joao Pedro scoring the winner in injury-time.
"We came here to win, unfortunately we didn’t win,” Onana told the club's website.
“We had a great game, I think we were better than them, we had more chances, we had more possession, we had more control of the game.
“Unfortunately, they disallowed us a goal, but that’s part of football, that’s part of life, we have to move on.
“Of course, we’re going to talk with each other, we’re going to sit and analyse the videos and what went wrong, and from there, we will move,” added the 28-year-old. “Big games are coming, big clubs, so the expectation is here, it will be very high.
“It’s (mentality) the key thing now, especially when you are playing for such a big club, and not only me, we have so many players who have that positive mentality and it is really helpful for the club and for the team.
“We move on, we are used to this, so no problem, we have to get ready again for the games that are coming.”