Enzo Fernandez gave Chelsea a first-half lead, but everything changed after Chalobah's dismissal on 53 minutes. Danny Welbeck, twice, and Martin de Cuyper struck for Brighton in the final 25 minutes to earn a famous win.
Maresca said afterwards: "It's a tough one because we were in control of the game, we scored one, we scored more, we didn't concede nothing, they just said to me also in terms of data, we had so many shots in the first half, 70% of possession, everything was fine.
"And then suddenly, because of our mistake, and then with the red card, again, the plan completely changed. So for me we played two games, first-half, and then unfortunately after the red card.
"For sure you cannot continue to make mistakes, also because they are big mistakes in terms of game changes, the red card at Manchester United and the red card today."
The Italian also said: "The message that we just said inside has been quite clear: against any team in the Premier League you cannot continue to give away presents, because what we are doing in this moment is to give away presents in terms of red cards, that is a big mistake because the game completely changed.
"For sure we need to learn, and we need to learn quick, so the reason why it can be a mix of experience or a mix of simple mistakes, then in terms of injury, I think that if you just watch first-half today, you don't think about the injured players, because first-half again we were in control, dominating the game, and then, again, I would like to tell you something different, but at the end it's the reality of the last two games, unfortunately with two red cards everything changed."