Munster flanker Chris Cloete has been ruled out for what the province describe as "the immediate future".
The South African sustained an ankle injury during Saturday's 31-20 bonus-point win over the Southern Kings in Port Elizabeth.
Cloete only lasted 8-minutes of the Southern Kings win, having been introduced as a 58th minute substitute for Tommy O'Donnell, and subsequently making way for Jed Holloway.
21-year old back-row forward Jack O'Sullivan has been flown to South Africa ahead of Friday's meeting with the Cheetahs in the Guinness Pro 14.
The Munster squad travelled to Johannesburg on Sunday, which they'll use as a base before Friday's game with the Cheetahs, and head coach Johann van Graan explained the call-up, "After an initial review on Chris Cloete’s ankle injury we’ve released him from the touring group as he won’t be available for the immediate future.
"In retaining a group of 28 available players we’ve brought in Academy back row Jack O’Sullivan, who arrived today."
The Cheetahs recently claimed the Currie Cup in South Africa, and Van Graan says the Ruan Pienaar-skippered side will be dangerous opposition, "This week is going to be even more difficult with the conditions and temperatures, playing at altitude against the Cheetahs.
"They are a team on form, and have been excellent in the Currie Cup. In the PRO14 so far they have dispatched Glasgow and Ulster comfortably at home, and hopefully we can learn from those games.
"From our point of view we have to focus on ourselves, and we’ve enough to work on following Saturday’s game.
"It’s only week three in the competition and there’s such a long way to go so we’ll take our points from the Kings game and move on.
Munster sit top of Conference B in the Pro 14 having picked up bonus-point wins in both of their games so far this season.