It was originally believed that O'Leary would not be back in action until late November or early December, and Munster team manager Shaun Payne was quick to praise the Corkman for his extraordinary recovery.
"Tomas has shown tremendous professionalism and dedication in getting himself back playing and great credit is due to him and to the work done by our medical and fitness staff," he said.
"It is, of course, great to see him back in the squad particularly after the injury he sustained."
O'Leary was not named in Munster's extended squad earlier this week, but Peter Stringer had picked up a calf strain in the game against Cardiff Blues last weekend and that did not respond well enough to treatment during the week to allow him make the trip.
Toby Morland will partner his old school mate Jeremy Manning at half-back for the Parc y Scarlets tie.
Coach Tony McGahan retains 21 of the 22 players that were on duty against the Blues, Marcus Horan missing out this time with French international prop Julien Brugnaut coming into the front row.
McGahan has made a second change up front, with hooker Damien Varley getting a chance to impress from the off.
MUNSTER (v Scarlets): Felix Jones; Doug Howlett, Barry Murphy, Lifeimi Mafi, Denis Hurley; Jeremy Manning, Toby Morland; Julien Brugnaut, Damien Varley, John Hayes, Donnacha Ryan, Mick O'Driscoll (capt), Alan Quinlan, Niall Ronan, Denis Leamy.
Replacements: Denis Fogarty, Tony Buckley, Billy Holland, Tommy O'Donnell, Tomas O'Leary, Scott Deasy, Tom Gleeson.