Roosters coach Trent Robinson might have found his insurance policy in 19-year-old Sean O'Sullivan if anything goes wrong with established halves Cooper Cronk or Luke Keary, as the NRL finals draw closer.

The teenage halfback stepped into the void of expectant father Cronk on Sunday, helping the Roosters to a 20-12 away win over the Gold Coast Titans.

Robinson said the 80 minutes of NRL football would have been enormous for O'Sullivan, topping off all the hard work on the paddock and in the gym.

"He grows in confidence training so much, as well as playing. That's where you get your confidence on debut – he's had a lot of time with Cooper and Luke and training with the other halves," Robinson told NRL.com.

"He's prepared for a long time, he gets his opportunity, and he delivers for us."

He scored a try beside the uprights, which lit up social media as fans believed O'Sullivan had knocked the ball on after it hit the goal-post padding.

So much debate around his first try could have had an adverse effect but Robinson said his young charge had a level head on his shoulders. O'Sullivan also had premiership-winning five-eighth Luke Keary as his roommate for the Gold Coast trip.

"It didn't knock his confidence," Robinson said.

"We had a brief chat about the exposure he was bound to get from his first game.

"He's an intelligent kid and been around football for a long time. It will be a different step for him but an important one to make." 

The performance also eased Robinson's concerns that if he lost one of his halves through injury, O'Sullivan has had a taste of first grade pressure.

"We've obviously got Mitch Cornish as well as Sean O'Sullivan in those positions and they keep pushing each other to fill that third spot," he said.

"Very important to have those players at this critical stage of the season. And that's why some of those guys, the other debutants [Paul Momirovski, Poasa Faamausili], played in those positions on Sunday.

"You want to have guys who are ready to go."

Keary thinks O'Sullivan displayed all the skills needed for an NRL No.7.

"He was really good. He's got a really loud voice, his kicking game is good, he's solid in defence. It's all you can ask for in a young half," Keary said.

"He just cruised around and led the team well ... very impressive for a young kid who can still play Under-20s."

Rookie Roosters halfback Sean O'Sullivan.
Rookie Roosters halfback Sean O'Sullivan. ©Scott Davis/NRL Photos

Winger Daniel Tupou was one of the first on the scene at Cbus Stadium to offer a congratulatory hug after O'Sullivan's maiden four-pointer.

"He's confident. The way he carried himself out on the field was great to see," Tupou said.

"He's trained with us in the pre-season and during the year, so he did well and all the boys were proud of him.

"The boys all have trust in him carrying the team around. God-willing nothing happens to Cooper or Luke, but he's there to fill in and do really well again."