Chelsea have spent about £185m on 11 signings this summer, adding to a squad that was already considered large.
Academy products Gallagher and Chalobah and striker Lukaku, who cost them £97.5m three years ago, are all training away from the first team.
Chelsea accepted a £33m bid from Atletico Madrid for Gallagher, who captained the club in 24 Premier League games last season.
However, he has been unable to complete his move for now after a deal for striker Samu Omorodion to go in the opposite direction collapsed.
Chalobah is being linked to several Premier League clubs, while Maresca has told England left-back Chilwell that he is allowed to leave this summer too.
Midfielder Carney Chukwuemeka and defender Axel Disasi were among the other players left out against City.
“I think every case is different,” said Maresca afterwards.
“Carney is a very good player. The problem is, because of some injuries in the past, I think the best for him is to play 30 or 35 games in a row, to show himself that he is fit and how good he is. With us, is he going to play 35 games? I’m not sure.”
Co-controlling owners Behdad Eghbali and Todd Boehly have spent about £1.5bn on players since taking over just over two years ago, following Roman Abramovich’s successful reign.
Redknapp, speaking on Sky, said: “Maresca’s biggest problem is trying to keep everyone happy. It is virtually impossible.
“Maresca will see so many unhappy faces. The best teams in the world work on 24 or 25 players in the squad. How or why did they think they can reinvent football? They are giving the manager an impossible job.”
If some of these issues sound familiar, former boss Graham Potter had similar challenges during his spell in charge.
The squad he inherited in 2022 was so big that the dressing room reportedly had to be reconfigured to fit more people in, and players had to change in the halls.
Chelsea had to leave Pierre-Emerick Aubameyang out of their squad for the Champions League knockout stages that campaign even after he had started all the group games becuase of the size of their squad.
And that will be another issue for Maresca, who will have to name a squad for the Premier League of 25 players (excluding under-21 players), with at least eight of those considered homegrown.