The 27-year-old was cited for an incident involving Ireland wing Tommy Bowe in his country's 40-9 defeat in Dublin on Saturday, an offence for which he received a yellow card at the time.
A disciplinary panel ruled that the 'tip 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.
The resulting six-week ban rules Pisi out of the European double header with Leinster, with the first tie taking place at Franklin's Gardens on Saturday, December 7 at 6pm and the return fixture scheduled for the RDS exactly a week later.
Northampton currently sit second in Pool 1, three points adrift of leaders Leinster after two rounds of action. The Saints started the campaign with a losing bonus point in defeat at French Champions Castres Olympique before bouncing back with a home win over the Ospreys last time out.
Pisi played in both those fixtures and would have been an almost certain starter for the clashes with Leinster had he been available. The former Auckland Blues and ASM Clermont Auvergne centre featured in all six of the Saints' Heineken Cup games last season, scoring four tries in the process. He has been involved in all but one of his side's league and European fixtures so far this season and has established a hugely powerful midfield partnership with new England squad member Luther Burrell.
Pisi, whose younger brother Ken also plays for Northampton, has the right to appeal.