Damian de Allende has withdrawn from the South Africa squad for their clashes with Wales and Scotland.
The Stormers centre suffered a medial knee ligament injury in their Super Rugby win over the Sharks on Saturday.
De Allende was taken for scans by the Springbok medical team on Monday and those have determined he will be sidelined for four to six weeks.
Lions fly-half Marnitz Boshoff will take his place - the uncapped player was one of those to miss out when coach Heyneke Meyer cut back his initial training squad.
"Despite already losing Jean de Villiers and Jaque Fourie to injury we still have good depth in the midfield which is why we decided to call up Marnitz," Meyer said.
"This is a great opportunity for Marnitz who I've known for a number of years.
"Everyone knows his goal-kicking is sublime but I was very impressed with the way in which he controlled the game in the Lions' victory over the Bulls at the weekend."
South Africa start their summer campaign against a World XV this Saturday before taking on Wales on June 14 and 21 - Scotland are their final visitors on June 28.