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.
Experienced internationals CJ van der Linde, Malcolm O'Kelly and Girvan Dempsey have been named on a strong replacements bench, but place-kicking centre Fergus McFadden misses out due to a calf injury.
In excess of 4,000 Leinster supporters are expected to travel to Toulouse this weekend as Leinster bid to qualify for their second successive Heineken Cup final.
Meanwhile, David Skrela will start for Toulouse at out-half in an all-French international back-line.
Skrela kicked seven penalties and three conversions in Toulouse's 42-16 quarter-final win over Stade Francais, and retains his starting place ahead of Jean-Baptiste Elissalde, Toulouse's top scorer in this season's tournament.
Toulouse coach Guy Noves has named the same back-line from the Stade match, but has made two changes to his pack.
Jean-Baptiste Poux drops to the bench as Springbok Daan Human gets a start at loosehead prop, while another South African Shaun Sowerby dons the number 8 jersey, relegating the highly-rated Louis Picamoles to the replacements.
Elissalde, who has been troubled by a calf problem, and the fit-again Maxime Medard will hope to have some impact off the bench in this battle of the European heavyweights.
TOULOUSE: Clement Poitrenaud; Vincent Clerc, Florian Fritz, Yannick Jauzion, Cedric Heymans; David Skrela, Byron Kelleher; Daan Human, William Servat, Benoit Lecouls, Romain Millo-Chluski, Patricio Albacete, Jean Bouilhou, Thierry Dusautoir (capt), Shaun Sowerby.
Replacements: Virgile Lacombe, Jean-Baptiste Poux, Yohan Montes, Yoann Maestri, Louis Picamoles, Jean-Baptiste Elissalde, Yann David, Maxime Medard.
LEINSTER: Rob Kearney; Shane Horgan, Brian O'Driscoll, Gordon D'Arcy, Isa Nacewa; Shaun Berne, Eoin Reddan; Cian Healy, John Fogarty, Stan Wright, Leo Cullen (capt), Nathan Hines, Kevin McLaughlin, Shane Jennings, Jamie Heaslip.
Replacements: Bernard Jackman, CJ van der Linde, Mike Ross, Malcolm O'Kelly, Stephen Keogh, Paul O'Donohoe, Eoin O'Malley, Girvan Dempsey.
Referee: Nigel Owens (Wales)
Assistant Referees: Andrew Small (England), James Jones (Wales)
Television Match Official: Graham Hughes (England)
Match Odds (Paddy Power): Toulouse to win: 4/11; Draw: 19/1; Leinster to win: 21/10
Pre-Match Quotes: Michael Cheika (Leinster) - "We're quite anxious for the game to get going. We've had a good preparation in the first few days of the week, obviously some of the guys got to stay at home at the weekend and do a bit of extra training to get prepared.
"Hopefully that will bear fruit on Saturday, but we know that we're going to face the sternest test of our whole time together.
"It's the biggest battle that we have ever come across and we'll have to dig in and fight hard because we're a couple of men down. To do that will take a real team effort.
"Before the loss to Connacht in Galway, we hadn't lost a game for four-and-a-half months. We don't like to lose but you've got to wake up to yourself sometimes and say you can always be beaten.
"If you bring the wrong attitude to any game of football, you're at risk to get beaten. I don't think that there's any fear of us bringing the wrong attitude to this game.
"It has been very low key as far as the preparation is concerned. You can see the determination in guys and the experiences of last year will help us because we know what it takes, not to win, we just know how hard it will be.
"This will probably be the toughest game that we've played together over the five years (I've been here). This is the biggest battle we've ever had, acknowledge that and get ready.
"Once you're ready for that, there's a good chance that you're going to perform to a level that gets you into a match and if you get a bit of luck too, you might win it."
Byron Kelleher (Toulouse) - "My goal is to win the Heineken Cup - and it is not just an individual dream, it's a dream of the team. I don't play competitions just to be a competitor, I play to win.
"The Heineken Cup is growing as a competition and continually puts pressure on you because you have to win away from home and the tournament has got stronger every year since I have been here.
"There used to be a mentality of focusing everything on winning at home and not worrying so much about winning away - that would be a bonus - but that has changed and French sides have adapted to professional rugby.
"It's great to be at home in the semi-finals and Leinster are a difficult team to play against, but these games are 50-50 and whether you are home or not it doesn't matter.
"Leinster are a very good team, they showed that in the quarter-finals, and Brian O'Driscoll is a world-class player and his performance against Clermont Auvergne was exceptional.
"We have Yannick Jauzion and if you look across Europe you won't see two better centres. There are big match-ups all over the field but that's what semi-final rugby is all about and why we play.
"Semi-finals are when your playmakers come into their roles and they have to make sure they take opportunities that come along, because you might only get one or two chances in a game.
"If you make wrong decisions you will miss those opportunities and it's about having a collection of people making the right decisions on the field - it's about leadership."
Pre-Match Multimedia -
Fogarty Geared Up For 'Massive Game'
Current Form - Last Five Matches - Toulouse - (Top 14): Won 34-3 at home to Montpellier; Lost 26-10 away to Biarritz Olympique; Won 28-23 at home to Racing Metro 92; Lost 15-13 away to Bourgoin; Won 25-17 at home to Castres Olympique
Leinster - (Magners League): Won 17-14 at home to Connacht; Won 16-15 away to Munster; Won 20-16 at home to the Ospreys; Lost 27-13 away to Connacht; Lost 30-6 away to Glasgow Warriors
2009/10 Heineken Cup Record - Toulouse: Pool 5: Won 36-17 at home to Sale Sharks; Won 23-19 away to Harlequins; Lost 15-9 away to the Cardiff Blues; Won 23-7 at home to the Cardiff Blues; Won 33-21 at home to Harlequins; Won 19-13 away to Sale Sharks; Quarter-final: Won 42-16 at home to Stade Francais
Leinster - Pool 6: Lost 12-9 at home to London Irish; Won 36-13 away to Brive; Won 32-7 away to the Scarlets; Won 39-7 at home to the Scarlets; Won 27-10 at home to Brive; Drew 11-11 away to London Irish; Quarter-final: Won 29-28 at home to Clermont Auvergne
2009/10 Heineken Cup Top Scorers - Toulouse: Points: Jean-Baptiste Elissalde 43; Tries: Yves Donguy 3; Leinster: Points: Jonathan Sexton 60; Tries: Gordon D'Arcy 4
Previous European Meetings: 7
Saturday, January 12, 2008 - Pool 6 - Leinster 20 Toulouse 13, the RDS
Sunday, November 18, 2007 - Pool 6 - Toulouse 33 Leinster 6, Stade Ernest Wallon
Saturday, April 1, 2006 - Quarter-Final - Toulouse 35 Leinster 41, Stade Municipal
Sunday, January 13, 2002 - Pool 6 - Toulouse 43 Leinster 7, Stade Municipal
Friday, September 28, 2001 - Pool 6 - Leinster 40 Toulouse 10, Donnybrook
Saturday, October 11, 1997 - Pool A - Toulouse 38 Leinster 19, Stade Ernest Wallon
Saturday, September 6, 1997 - Pool A - Leinster 25 Toulouse 34, Donnybrook
Heineken Cup Records:
2008/09: Reached the quarter-finals
2006/07: Failed to qualify from Pool 5
2005/06: Reached the quarter-finals
2001/02: Failed to qualify from Pool 6
2000/01: Failed to qualify from Pool 3
1999/00: Reached the semi-finals
1998/99: Reached the quarter-finals
1997/98: Reached the semi Finals
1996/97: Reached the semi-finals
2007/08: Failed to qualify from Pool 6
2006/07: Reached the quarter-finals
2005/06: Reached the semi-finals
2004/05: Reached the quarter-finals
2003/04: Failed to qualify from Pool 3
2002/03: Reached the semi-finals
2001/02: Reached the quarter-finals
2000/01: Failed to qualify from Pool 1
1999/00: Failed to qualify from Pool 1
1998/99: Failed to qualify from Pool A
1997/98: Failed to qualify from Pool A
1996/97: Failed to qualify from Pool B
1995/96: Reached the semi-finals