Sexton was given every chance to prove his fitness but Leinster coach Michael Cheika said that playing him would have been 'too big a risk' and that the management also have 'the player's welfare and safety in mind.'
Shaun Berne will deputise at out-half for the province, and Cheika has every confidence in his fellow Australian who donned the number 10 jersey for Leinster's back-to-back pool wins over the Scarlets in December.
In all, Leinster have made two changes in personnel to the team that edged out Clermont Auvergne in the last round.
The second change sees loosehead prop Cian Healy brought back into the front row in place of CJ van der Linde, with Stan Wright moving across to the tighthead side of the scrum.
In a significant boost, Rob Kearney has recovered from an ankle injury to start at full-back in a back-line which includes five Ireland internationals.
Shane Horgan and Isa Nacewa make up the rest of the back-three with Brian O'Driscoll partnering Gordon D'Arcy at centre. Berne and Eoin Reddan form the defending champions' half-back partnership.
Leo Cullen leads the side and starts in the second row with Scotland's Nathan Hines, in front of all-Ireland international back row of Kevin McLaughlin, Shane Jennings and Jamie Heaslip, the man-of-the-match from the quarter-final win.