Fla thumped Chelsea 3-1, with Nicolas Jackson seeing red just six minutes after coming on in the second-half. The Senegal international was sent off for a nasty challenge on Lucas.
Chelsea had the lead through Pedro Neto before being overrun by the Brazilians and Maresca said afterwards: "Six minutes that completely changed the dynamic of the game.
"I think, until their first goal, we had started the second half much better, but then two goals in two minutes and the red card changed the dynamic of the game."
The Italian continued: "In the last two days, we tried something completely different. We played in a different structure today to prepare for next season and have more options.
"The plan was quite good for an hour, but then we conceded the goal, and the second one, and then we had the red card. In the end, with us down to ten men, they deserved to win the game."
Chelsea now meet ES Tunis on Tuesday and Maresca added: "We need to win on Tuesday to make sure we get through to the next stage."