Racing were hit with an early injury, as outhalf Pat Lambie was forced off with injury after only 2 minutes.
Carter is playing his final European match for Racing before heading to Japanese club rugby, but Leinster fly-half Jonathan Sexton insists that even at 36 years old the World Cup victor remains a threat.
McGrath's return sees Jamison Gibson-Park drop to the replacements bench, which means New Zealader Lowe will be forced to watch on as a spectator in Bilbao.
"I know this is a huge desire of his to win the Champions Cup and I'm sure he wants to win the Top 14 again as well".
Their anguish was soothed slightly due to a penalty resulting from a Garry Ringrose high tackle in the ensuing passage of play, and Iribaren sent it spiraling through the posts to give his side the lead.
While Ireland stand-off Johnny Sexton starts at 10 for Leinster, Carter has fallen down the pecking order at his Parisian side.
He joined big-money French side Racing in 2015 and is leaving to play for Kobe Steelers in Japan.
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Sexton and Iribaren traded penalties but also both traded misses from the half way line, Sexton twice. Racing were beaten by Munster and Castres in the group stage but four victories sent them through as one of the best runners-up.
Despite Cullen's eagerness to paint Leinster as the underdogs who punch above their weight, Racing's France number eight Yannick Nyanga points out they have been backed to win by bookmakers.
"The depth of their squad is phenomenal".
Three-time winners Leinster look to preserve their ideal European Champions Cup final record as they meet Racing 92 in Bilbao.
Racing must decide whether to start legendary All-Blacks out-half Dan Carter or use him from the bench.
"They have assembled an expensive bunch of players and we have to do things fairly differently at Leinster by producing guys from within mainly, although we bring in a couple of key signings from overseas".