The 41-year-old Du Plessis injured his thumb while fielding against MI Cape Town on Saturday at the Wanderers and did not come out to bat in the run-chase.
Super Kings head coach Stephen Fleming said after the game: "He got his thump jammed in the ground which has done some damage.
"Obviously enough damage not to bat which was [not good], when you are chasing 200 you need your star players to be out there.
"So, to lose him is another factor but we'll take into consideration what happens going forward, we are hoping it's bruising and settles down but I don't think I feel overly optimistic with the amount of injuries we've had so far.
"A bit of a pessimistic view on these things, so I'm hoping for the best but we may have to prepare for other means."
Leus du Plooy has been added to the Super Kings' squad for the remainder of the campaign.
The Super Kings currently sit third in the standings after winning three of their seven encounters so far this season.
Aiden Markram has been one of the standout performers of the 2026 T20 World Cup, and former Proteas captain Faf du Plessis believes his transformation as an opener began during his time in the Indian Premier League.
Sunrisers Eastern Cape underlined their status as SA20's dominant force with a nerve-shredding six-wicket victory over Pretoria Capitals at Newlands, clinching a third title in four seasons.
Sunrisers Eastern Cape sailed into yet another SA20 final after beating the Paarl Royals by seven wickets in a low-scoring encounter at the Wanderers on Friday.
Lhuan-dre Pretorius found his form at the perfect moment as Paarl Royals ended Joburg Super Kings' SA20 campaign with a commanding 36-run win at Centurion on Thursday night.
