Zabaleta was taken off with a hamstring injury after 12 minutes of City's 3-1 League Cup win over Leicester City on Tuesday.
It added to a growing injury list for Pellegrini, with leading goalscorer Sergio Aguero also out of action, but the Chilean is confident Zabaleta could be back within two weeks.
"I think Pablo is not so big an injury. Maybe he will be back soon. Maybe a week or two, but the doctor will decide," he said on Friday.
"Aguero could be (out for) eight weeks, (or it) could be four. We will see the way he is working, but he is improving every day.
"Micah Richards, Matija Nastasic and Stevan Jovetic will also miss this (Fulham) game."
City face a Fulham side that have shown signs of improvement under new head coach Rene Meulensteen and Pellegrini is not expecting an easy match at Craven Cottage.
"For different reasons, it is always very hard to play all teams in the Premier League, but the important thing is the way we play the game," he added.
"I’m absolutely sure that our mentality must work, thinking that we always have a strong team in front of us."
