Chelsea had the lead through Joao Pedro, but La Dea fought back after the break and struck through Gianluca Scamacca and Charles de Ketelaere.
Afterwards, Maresca admitted: "After the goal we conceded, we lost control. Second-half, we had two good chances to score the second one. When we conceded the 1-1, we lost some concentration. We can avoid both of the goals we conceded. They're easy goals.
"I think tonight's first XI, we had eight or nine players that played against Tottenham, Barcelona, Wolves, Arsenal. If you see the five changes we did compared to Bournemouth, it's different.
"For sure, we conceded two goals that for me we can avoid. When you concede, the first one is the moment when you can't lose the control of the game but I have a feeling that we lost the control of the game after the goal we conceded."
Maresca also considered what Chelsea need to finish in the competition's final eight.
He said, "Probably we need two wins. Sixteen points might get you top eight, I'm not sure. The next one, for sure, if we want to finish top eight, we need to win both.
"Yes, we are aware (of how tough the fixture schedule will get if they are in play-offs), absolutely. Now, the game is finished. The focus is on Saturday's game. We need to win games."