Noni Madueke and Christopher Nkunku scored to give Chelsea a 2-0 first-leg lead.
Maresca later said: "I was a bit worried because in the last 10 minutes we played a little bit with fire because we could managed the game completely differently. It was good that we didn't concede but we can learn.
"Noni is a player I like. He is doing well but the thing he has to improve is consistency every day and work hard and then he will get minutes for sure.
"The game first half was for Marc Guiu. There was a lot of space for him to drop and give us an extra man. He struggled a lit bit. The intention was to give him minutes but he had to behave as a midfielder.
"I told him he probably doesn't like easy goals, he likes more difficult ones. He is the youngest in the squad, he works for the ball, runs a lot presses...he is going to score for sure."
On Mykahylo Mudryk, he also said: "This is the mission, this is Mudryk, not only tonight but since he joined the club. I think he had good moments but when he has the ball you don't know if you'll get one thing or another. It is consistency. We are going to try and help him to change. When he is in the final third it is is taking the right decision. Most of his mistakes are about the choice."
On a potential injury for Cole Palmer, Maresca added: "Cole felt something but he looks fine. Hopefully it is no problem and he can be available for Sunday's game (at Wolves).
"We are learning how to manage the game. Keeping the ball is the correct intention at 2-0 up but where is not good, in our half. We need to do the same on the opposite side. It was important to win the game with nine changes. There were many good things."