Enzo Fernandez had Chelsea had on nine minutes, but a double from Marco Asensio saw Villa win 2-1 on the night.
Maresca said afterwards, "This is, for me, probably the toughest defeat of the season. The reason why is because the reaction from Brighton was brilliant, the performance was very good and unfortunately, in this moment, it is not enough. That's why it is a tough one.
"I don't think we deserved to lose the game, to be honest. In this league, you have to be clinical. The difference most of the time is inside of the box. Unfortunately at 1-0 up we had some chances and didn't score, at 1-1, we had some chances and didn't score. In the end, anything can happen and unfortunately we lost the game."
Filip Jorgensen's error led to Asensio's winner and Maresca also stated: "The 'keeper position is a delicate one because the striker can miss the goal and sometimes it doesn't look so important but when a 'keeper makes a mistake, it's so clear and everyone can see that. This is a moment where we need to support each other, be strong and continue to work hard.
I have a feeling in this moment that we need to win a game to go again. But now, the good thing in two or three days we have one more game at home and hopefully we can finally, with our fans in our stadium, try to get the three points and from there, start again and try to go again."
Asked if Jorgensen will maintain his place for the next game against Southampton, Maresca said: "I don't know. In this moment, it is not today's mistake that I am going to change my mind about Filip in this moment. We will see."
He concluded: "The reason why I said before that it is the toughest defeat of the season is because today, again, our team was there. You can recognise: off the ball, aggressive, on the ball, tried to play and create chances. Without a No.9 in the last two games, we struggled to create chances.
"Today, we tried Pedro during the week, it worked very good, attacking in behind and creating chances, scored a goal but it is a moment where we probably are a bit unlucky. We need a win and try to go again."