Eben Etzebeth has been ruled out for six to eight weeks after suffering torn ligaments in his left ankle during a pre-season game against Boland.
Springbok forward Etzebeth limped off in the first half of team's 57-0 pre-season win on Saturday.
"It's unfortunate for the team and Eben, but like I said after the match yesterday injuries are part and parcel of the game," said Stormers coach Alistair Coetzee.
"We are fortunate to have players in our squad who can step in and fill that role and that's our focus for now - to find a player to fill that No.4 lock role.
"At the same time, however, our medical team will also ensure that Eben recovers well and comes back strongly for the Stormers."