Ronan O'Gara is assistant coach at Racing 92 as they take-on Mark McCall's Saracens side in the showpiece event of European club rugby.
Kick-off in Lyon is at 4:45pm.
Meanwhile, Conor O'Shea's time with Harlequins as their rugby director has ended in defeat.
His side were beaten by Montpellier in the Challenge Cup final last night by twenty-six points to nineteen.
The match was refereed by Tipp Town man John Lacey.
