The Argentinean was called off the bench in the goalless draw with Aston Villa on Monday after Arthur Masuaku was sent off.
He slotted in at left-back for the final 23 minutes of the game at Villa Park and ensured the Hammers held on for a point and their third successive clean sheet across all competitions.
Zabaleta's opportunities at West Ham are set to be limited this term with Masuaku and Ryan Fredericks the first choice options on either side of defence.
But he could be set for a first start of 2019-20 on Sunday in the absence of Masuaku when Manchester United travel to the London Stadium.
The 34-year-old, who has already confirmed this will be his last season in England, insists he has been keeping himself fit and is ready to deputise if required against the Red Devils.
Zabaleta told the club's official website: "This will be my last season in England, so I'm just enjoying every training session, every single minute on the pitch.
"There are plenty of games this season, so I need to be ready for every opportunity. The whole team has to be ready and fit.
"As a player you always want to be in the starting eleven in every game, but also when you are on the bench you have to support the players who are playing at the moment."
Despite Zabaleta's willingness to step up this weekend, Pellegrini may opt to bring in left-back Aaron Cresswell. The 29-year-old has only featured once this season and wasn't involved in the matchday squad at Villa earlier in the week.
But Pellegrini insists he is "absolutely confident" about his ability to offer cover.