Burley: Onana needs to take full responsiblity for Man United's result (2:35)The "ESPN FC" crew react to Manchester United's 3-3 draw at Galatasaray that puts them on the brink of Champions League elimination. (2:35)
Asked if as captain he needed to talk to Onana, Fernandes said: "He knows how to deal with these moments. He has to do better on the pitch as we all do. We should have done more to have taken a better result. It's not just because of him that we got this negative result. We all have to take responsibility for what we do, me first."
United manager Erik ten Hag backed Onana after the game, saying: "It is not about individuals... it is always about the team."
Fernandes agreed: "We all need to know that we have to improve and that there are things to improve. We are making many mistakes in the Champions League and it's costing us."
Only Antwerp have let in more goals, 15, in the Champions League than United (14) this season.
André Onana conceded two free kicks in the Champions League draw with Galatasaray on Wednesday. BSR Agency/Getty Images
"We've conceded a lot of goals up to now," Fernandes said. "We are probably the team that concedes the most goals. We have to control the game better."
Fernandes was also disappointed that his team did not kill the game off when they had a chance having led 2-0 and 3-1.
"We have to be more clinical," he said. "It can't be that with all the clear opportunities that we had, we didn't make the most of it and ended up getting a negative result. We should have scored more goals."
Fernandes said he knows his team's chances of playing in Europe in the new year are slim.
"There is very little we can do but to win our next game against Bayern Munich and hope for a good result in the other game to go through," Fernandes said.
"Unfortunately, we depend on others. It could have been otherwise had we gotten a win but we weren't good enough to achieve that."