Ugo Monye, dropped after the first Test defeat, returns in the number 14 shirt, with world player of the year Shane Williams starting instead of Luke Fitzgerald on the other wing.
England props Andrew Sheridan and Phil Vickery take over from injured pair Gethin Jenkins and Adam Jones, while back-row changes see starts for Wasps flanker Joe Worsley and Wales openside Martyn Williams.
The seven changes and one positional alteration mean just four players will have started all three Tests - fly-half Stephen Jones, scrum-half Mike Phillips, skipper Paul O'Connell and number eight Jamie Heaslip.
"It is fair to say injuries have had an impact on the tour, as you would expect," said McGeechan.
"We were unfortunate to lose Brian O'Driscoll, Jamie Roberts, Gethin Jenkins and Adam Jones through injury last Saturday.
"However, the players coming into the starting XV are all talented international players and are as keen as ever to turn the tables on the Springboks.
"Last Saturday, we were minutes away from winning, and that is a huge positive to take into the third Test."
McGeehan has opted for five forwards on the bench - hooker Ross Ford, prop John Hayes, lock Alun-Wyn Jones, plus flankers Tom Croft and David Wallace, who drop out of the starting XV.
Scrum-half Harry Ellis and fly-half James Hook will provide back division cover.
McGeechan added: "The 2009 Lions have thrown themselves into the spirit of the tour, interacted with the people of South Africa and shown themselves to be great ambassadors for northern hemisphere rugby.
"As a squad, they have met many challenges on the tour, including playing two matches per week, the inevitable injuries and two narrow Test losses.
"Despite this, they have remained a tight, focused squad."
Three players - Sheridan, Worsley and Martyn Williams - all make their full Lions Test debuts.