Chelsea will be without Liam Delap for '10-12 weeks', according to Maresca, after the striker sustained an injury before the international break.
Delap lasted just 14 minutes of Chelsea's recent victory over Fulham, meanwhile Cole Palmer did not feature at all for the second game running.
The England attacker did not make Thomas Tuchel's Three Lions squad during September's international window, due to injury.
Cole Palmer still rated doubtful for Chelsea's visit to Brentford
Palmer's issue appears to have been alleviated to an extent but Maresca's latest fitness revelation suggests the playmaker may not be ready to start, or complete 90 minutes.
"Cole took part in the session yesterday for the first time," Maresca revealed on Friday morning. "Not the entire session. We have one more session this afternoon, and we are going to try with him to see if he's OK.
"Otherwise, he will be out tomorrow. He is recovering from an injury - it's not managing him. When he is available, we need to manage him because of the amount of games."
Palmer played 90 minutes in Chelsea's Premier League opener against Crystal Palace but has not appeared since.
Ex-Manchester City duo Palmer and Delap may be suffering from the effects of a FIFA Club World Cup hangover, having featured heavily for the Blues at the summer tournament in the United States.
Stamford Bridge teammate Levi Colwill has already been ruled out for an extended period of time with a serious knee injury picked up on his return to pre-season training last month.
Chelsea make the short journey across West London to face Brentford on Saturday evening, potentially without star man Palmer in the starting lineup.
The Blues sit second in the Premier League table with two wins and a draw from their first three matches.
