Pisi appealed the ban for the hit on Ireland wing Tommy Bowe during his country's 40-9 defeat in Dublin two weeks ago but an independent committee, chaired by Professor Lorne Crerar, found no reason to alter the initial verdict.
The 27-year-old remains unavailable for selection until December 23, nine days after the second of the mouth-watering match ups with the triple Heineken Cup kings at the RDS.
The original disciplinary panel ruled that the tackle warranted a red card and decided that it merited a mid-range entry point of an eight-week suspension. That was then increased by two weeks as a deterrent with regard to tip tackles, before four weeks were taken off after various mitigating factors were taken into account.
Pisi, who pleaded guilty to the charge, had hoped to have the ban reduced but he is now resigned to missing Rounds 3 and 4 of continental competition and the Aviva Premiership fixtures against Newcastle Falcons, Worcester Warriors and London Wasps.
His first game back in the fold will be the December 28 tie with Bath at Franklin's Gardens, meaning he will at least be available when the Heineken Cup returns again in the New Year.