"Sport can be very cruel," said McGeechan, who will also be required to name a replacement for injured centre Tom Shanklin.
"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 the Scarlets in Cork.
"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 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," added McGeechan.