While offering little detail, Maresca admitted today: "We have had a problem in the last two days with Levi Colwill and we don't know how long he is going to be out so we will see. Hopefully not too long but it's a problem for us."
On what sort of problem Colwill has, the manager continued: "I'm not sure. We have to wait."
On whether the injury means they will have to make another move in the transfer market, Maresca said: "It depends. We have to wait and then we will see.
"We know each other from last year and we know how important Levi has been to us. I spoke with him yesterday and I told him if we achieve last year it will be because of him - he has been a main player for us."
On how the injury occurred, Maresca added: "It was the first session on Monday in the last minutes of the session. He felt something. We have to wait and then we will see."
Meanwhile, Maresca also discussed new signing Jorrel Hato. He said: "He is much more mature than his age.
"He is from a different country, a different league with a different tempo and intensity so he needs to adapt but for sure there is no doubt that he is going to help us."
Chelsea meet Bayer Leverkusen on Friday night at Stamford Bridge.