British and Irish Lions head coach Ian McGeechan has called up Scotland scrum-half Mike Blair to replace the injured Tomas O'Leary.
The Munster and Ireland scrum-half suffered a fractured ankle in April while playing for the Magners League champions Munster against Scarlets.
McGeechan offered his sympathy to O'Leary after selecting Edinburgh star Blair.
"Sport can be very cruel. One minute Tomas was celebrating his well earned call up to the Lions tour to South Africa, the next he is being carried off injured while playing for Munster against Llanelli Scarlets in Cork," he said.
"Unfortunately the injury was a serious one and it ruled Tomas out of the tour. We wish Tomas all the very best for a quick recovery.
"The selectors and I have had plenty of time to review the situation in terms of a replacement scrum half and were in agreement that Mike Blair should replace Tomas. He was on our reserve list for such an eventuality.
"Mike is an excellent addition to the British and Irish Lions squad.
"As a national captain he is well respected by his peers and his 58 Tests for Scotland mark him as a very capable and experienced scrum-half."
Cardiff Blues centre Tom Shanklin has also been ruled out of the tour after dislocating his shoulder but no replacement has yet been named.