"While the injuries are being treated by the medical team, we felt it prudent that we call up Gordon to provide us with extra cover in midfield.
"Due to the short nature of this tour we have to be prepared for every eventuality.
"An extra back at this stage - three matches into the tour - will provide the extra depth to cover the existing injuries plus any further injuries we may encounter.
"Gordon showed in the Heineken Cup final and the recent Barbarians match against England that he is back to his best.
"He was on our reserve list and we had no hesitation in calling him up to the squad as he is an extremely experienced player who has played 41 times for his country.
"He was also a British & Irish Lion on the 2005 tour to New Zealand where he played six provincial matches.
"He earned a Lions Test cap in the match against Argentina in Cardiff prior to that tour.
"His midfield partnership with Brian O'Driscoll for Leinster and Ireland is a proven one and can only benefit the Lions squad here in South Africa," McGeechan added.
D'Arcy, a Grand Slam and Heineken Cup winner this season, will arrive in South Africa on Thursday (June 4) and will meet up with the squad in Johannesburg.