United need Fernandes to convert an injury-time penalty to secure the points at Falmer stadium.
"It was a difficult game. Brighton have a really good team," Fernandes told BT Sport. "In the first-half, we gave them too much space to play. We were not as aggressive as I wanted. We gave away two goals. The penalty I can do much better, the second goal we need to do much better also.
"We made some mistakes but we needed to believe until the end. We created chances, okay it was a penalty but we don't know whether when [Maguire] headed the ball, maybe it goes at goal without the handball. We have to do much better looking forward against Brighton again."
He added: "Brighton did very well today but maybe because we were not as aggressive as we wanted. We need to recover more balls, press better, we need to keep the ball better. They had a lot of chances - true. Maybe they deserved more but the point is to score goals, not hit the crossbar or the post. Sometimes it happens.
"In the games against Sevilla and Copenhagen, we had 40 shots but only scored twice from penalties. Sometimes you need luck. You need your defenders defending well and your goalkeeper. David [de Gea] did very well. We can do much better for the goals, everyone will agree and we'll work on that."