The 20-year-old, who made two appearances off the bench in earlier rounds, dons the number 6 jersey, with the in-form Sean O'Brien shifting to number 8.
Leo Cullen leads the side for the visit of Saracens, partnering Scotland international Nathan Hines once again in the second row.
With Eoin Reddan linking up with Jonathan Sexton at half-back, there is a familiar look to the remainder of the back-line as Gordon D'Arcy and Brian O'Driscoll form a well-oiled centre partnership.
Fergus McFadden joins Leinster's most capped player in Heineken Cup history, Shane Horgan, and Isa Nacewa in the back-three.
Ireland internationals Isaac Boss, Luke Fitzgerald and Kevin McLaughlin have been named amongst the replacements.
Commenting on the loss of Heaslip, head coach Joe Schmidt said: "We met on Sunday and I said to Jamie, 'look you're not going to be in this week.'
"After Monday's training we gave him one more chance. It went better but, with the bone bruising, he was pretty uncomfortable on Tuesday.
"Sean's a really different player to Jamie. He's a bit younger and is probably a rougher diamond. But potentially, yeah, he could become world class.
"I really like Sean as a player, the players that have played with him really like what he brings to the table. He has a real robustness about him and he has a massive work ethic.
"Sean's also underestimated in the air. He's a very good aerial player and he really demonstrated that against Racing Metro.
"He's quite versatile. He can play anywhere in the back row. And he makes good bread too!"
Meanwhile, anyone attending Saturday's match at the RDS has the chance to avail of a very special, one day only offer where supporters can buy two tickets for the Magners League match against Aironi on Thursday, February 10, and get two junior tickets for free.
The full price cost for a family of four for this match would be €60, but on Saturday it will be only €40, which is the equivalent of having two children (Under-18) going for free.
LEINSTER (v Saracens): Isa Nacewa; Shane Horgan, Brian O'Driscoll, Gordon D'Arcy, Fergus McFadden; Jonathan Sexton, Eoin Reddan; Cian Healy, Richardt Strauss, Mike Ross, Leo Cullen (capt), Nathan Hines, Dominic Ryan, Shane Jennings, Sean O'Brien.
Replacements: Jason Harris-Wright, Heinke van der Merwe, Clint Newland, Devin Toner, Kevin McLaughlin, Isaac Boss, Ian Madigan, Luke Fitzgerald.