The 25-year-old O'Leary has been ruled out of action for six months after dislocating and breaking his left ankle in his side's 29-10 Magners League win over the Scarlets on Friday night.
"It was a phone call I didn't want," said McGeechan after hearing the news of O'Leary's injury. "I'm obviously desperately disappointed for the player.
"But we'll take our time and keep our eye on things. I won't rush in naming a replacement .
"I'm not looking to name a replacement for 10 to 14 days. There's some rugby to watch and I'll take my time."
McGeechan's decision to wait before naming the scrum half who will join Mike Phillips and Harry Ellis in his squad is partly conditioned by the fixture list. This weekend features the European semi-finals, with only Northampton's Ben Foden and Munster's Peter Stringer likely candidates from the four games in the Heineken Cup and Challenge Cup.
Northampton face Saracens in the Challenge Cup and Munster meet Leinster in the Heineken Cup. The following weekend will see Leicester face Bath and Harlequins meet London Irish in the semi-finals of the Guinness Premiership.
Those games will allow McGeechan and his coaches to watch England's Danny Care try to push Harlequins into the Premiership Final.
Other names that could come into the frame are 2005 Lions Test scrum half Dwayne Peel, Scottish skipper Mike Blair, who steered his Edinburgh side to a 32-30 away win over the Ospreys in the Magners League on Saturday, and possibly even 2005 tourists Chris Cusiter and Gareth Cooper.