He changed the quarter-final against Clermont and the game changed a lot when he came on against Munster.
Pat Lambie then sustained a knee injury in the second minute, so French global Tales, who did not feature on the team list in the morning, had to deputise.
A high Garry Ringrose tackle following Lambie's injury saw Teddy Iribaren, deputising for the injured Maxime Machenaud at scrum-half, open the scoring with a penalty in a tight game where defence was the victor and wide, expansive attack never an option.
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.
Racing 92 stand between Leinster and a record-equalling fourth European crown with Cullen eager to highlight their contrasting approaches to scaling the pinnacle of the club game in this hemisphere.
Racing's giant Fijian lock Leone Nakarawa was lucky not to be shone a yellow card after a deliberate knock-on with Leinster again on the front foot.
Captain Isa Nacewa sealed victory with a penalty as only 90 seconds remained on the clock.
Iribaren nailed his third penalty five minutes into the second period as Racing tested the Irish defence.
The sides were level 6-6 at the break.
Racing conceded 13 penalties in comparison to Leinster's 10 with both sides missing their share of goal kicks.
Leinster responded immediately, Australian flanker Scott Fardy exploding down the wing.
Tales was a late replacement for substitute fly-half Dan Carter - who withdrew before the game with a hamstring injury - and was brought into the match after just three minutes when first-choice fly-half Pat Lambie went down with a knee injury.