The 21-year-old was on the verge of breaking into the Cheetahs Super 14 squad this season when he was struck by a knee injury that required an operation. He was on the way back to full fitness, and a likely spot in Naka Drotske's Vodacom Cup squad, when he was involved in the accident.
Pietersen, who played for South African Schools, Under 19 and Under 20, made his Currie Cup debut for the Cheetahs last season and had been competing for the No 15 jersey with Hennie Daniller.
The Cheetahs meet the Lions in Bloemfontein on Saturday, 6 June. They have lost all six of their Super 14 games to date this season and are eight points adrift of their nearest rivals, the Lions at the bottom of the table.