The Blues have made 11 signings so far this summer which has seen the senior squad swell to more than 40 players.
Earlier this week, Maresca said England internationals Raheem Sterling and Ben Chilwell were part of a group of about 15 players told to train away from the main first-team group.
“At the moment, with the squad that we have, I am working with 22, 23 or 21 players – not with the 42 players, otherwise it is impossible,” Maresca said.
“It is impossible for any manager in the world to make a session with 45 players. It’s impossible. You cannot do that.”
Sterling was left out of Maresca’s first Premier League squad, for the 2-0 home defeat by Manchester City on Sunday, while he also missed their Europa Conference League play-off first-leg win against Servette on Thursday.