O'Leary has now had surgery on the injury and will be unavailable for selection for between six and eight weeks.
The 26-year-old will miss Saturday's Heineken Cup opener at London Irish and the Round 2 clash with Toulon, as well as up to three Magners League ties.
He is now a serious doubt for the start of the international calendar too, with only the last of Ireland's autumn Tests falling more than eight weeks away from today.
The news is a further blow for the 2009 Magners League Champions after they saw centre Lifeimi Mafi handed a seven-week suspension that rules him out of action until November 22.
But the Red Army have strength in depth at scrum-half and can call on veteran Ireland international Peter Stringer and 24-year-old Duncan Williams to fill the void.
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