Mascherano injured a calf muscle in Sunday's Premier League draw with Arsenal while Agger suffered a concussion in the same game.
"Javier will always be in my plans until the day comes when he leaves the club," said Roy Hodgson. "And for me that will be a long time in the future if I can get my wish. But he's injured at the moment and won't play tomorrow.
"He came off after about 80 minutes on Sunday with a slight pull in his calf muscles and has only just started to jog today. The worry for us is that he won't be fit for Monday (against Manchester City)."
Lucas or new signing Christian Poulsen are most likely to take over from Mascherano in midfield while Fabio Aurelio is in contention for Agger's spot at left-back.
Diego Cavalieri will provide back-up to goalkeeper Jose Reina as Brad Jones' move from Middlesbrough was not completed in time.
Star striker Fernando Torres is pushing for a starting place after appearing from the bench against Arsenal on his return from a groin injury.
Liverpool (from): Reina, Johnson, Carragher, Skrtel, Aurelio, Kuyt, Lucas, Gerrard, Cole, Jovanovic, Ngog, Aquilani, Torres, Pacheco, Krygiakos, Rodriguez, Babel, Wilson, Spearing, Poulsen.