The Ireland international had been cited for making contact with the eye area of Leinster skipper Leo Cullen during a fiery European encounter at Croke Park and has now been suspended until September 9.
Even though the seriousness of the offence was considered to be at the lower end of the scale, the length of the ban means Quinlan will be unavailable for all 10 games on the forthcoming Lions tour of South Africa.
Quinlan has the right to appeal the decision but, if the verdict is upheld, McGeechan will be forced to make his third change to his original tour squad before the Lions have even departed these shores.
The Lions have already lost Tomas O'Leary and Tom Shanklin to injury since the original squad announcement on April 21 and now they are likely to be looking for another back row player to replace many people's surprise inclusion in the first choice party.
The leading contenders if Quinlan is ruled out are thought to include Wales skipper Ryan Jones and England's young star Tom Croft, although the fact that McGeechan is still yet to name a replacement for Shanklin suggests that a decision will not be imminent.