Inside the opening 20 minutes of the game, with the Titans leading four points to nil, Titans' captain and star Queensland forward Tino Fa'asuamaleaui charged into the line, appearing to raise his forearm into a collision with Koroisau, who was in defence.
Koroisau would leave the field a minute later with Fox Sports sideline commentator Corey Parker suggesting he had bloody around his face.
Here's the exact moment Api Koroisau suffers a suspected broken jaw.
— Fox League (@FOXNRL) June 8, 2023
Parker confirmed minutes later that the Tigers' bench and medical staff were anticipating a fractured jaw for Koroisau.
“There is a suspected fractured jaw to Apisai Koroisau, massive news with Origin just around the corner," Parker said.
A fractured jaw can leave a player on the sidelines for as long as two months, which could rule Koroisau out of the remainder of the Origin series.
Koroisau was taken to hospital, and coach Tim Sheens confirmed post-game that he had broken his jaw in two places.
"It's a major issue. Surgery tomorrow. He will be minimum six weeks if he is lucky," Sheens said in his post-game press conference.
Raised forearms have been a controversial topic in recent seasons from attacking players, with penalties applies appearing inconsistent at best.
Fa'asuamaleaui, nonetheless, will face a nervous wait until the match review committee unveil their penalties from the game on Friday morning after he was put on report for the incident.
Tigers' coach Sheens said it looked like an elbow.
"It looked like an elbow to me, but I won't make any real fuss about it until I see it [properly]," Sheens said.
Fa'asuamaleaui, speaking post-game, said it was simply a contact sport.
"It's a contact sport. I'm running as hard as I can and he is obviously getting up fast trying to stop me. I guess he came off second best but I'm hoping he is alright, and I'm hoping I'm alright too," Fa'asuamaleaui said.
This incident moves the Titans' captain to a third and subsequent offence under the NRL's judiciary code. While he will still escape with a fine for a Grade 1 offence, if he is charged with Grade 2 or above, it will result in a suspension unless he is able to fight at the judiciary and be freed.
That would leave him out of Origin 2, with no games remaining between now and teams being named early next weekend - Queensland are expected to announce their side on Monday morning, while the Blues will wait until Tuesday following the Parramatta Eels and Canterbury Bulldogs' Kings Birthday clash.