Matterson and O'Sullivan in line to replace injured Keary

Rookie Sean O'Sullivan or experienced utility Ryan Matterson look to be the frontrunners to replace Luke Keary in the Roosters line-up for Saturday night's game against the Cowboys at Allianz Stadium.

Keary left the same venue on Sunday with his left knee in a brace after the comprehensive 36-18 win over St George Illawarra.

He is booked in for scans on Monday and coach Trent Robinson is hopeful his time on the sidelines won't be lengthy.

"He's pretty good. I think it's a medial [ligament] so he's got the brace on as a precaution," Robinson said.

"I don't think it's anything too bad. But I don't know how long it will take to turn it around."

When pushed that the sight of Keary being helped up the tunnel after the 51st-minute injury made it unlikely he'd be able to back up in six days, Robinson said: "You'd think so.

"I've been wrong before with players and playing the following week so we'll just wait and see.

Luke Keary shows the pain after suffering his knee injury.
Luke Keary shows the pain after suffering his knee injury. ©Grant Trouville/NRL Photos

"When a player, an outside back, comes off there's usually something wrong especially a player like Luke as I don't think he's come off very much at all.

"So it's obviously a concern, but also seeing him afterwards I don't think it's a long-term concern."

Robinson also wouldn't hazard a guess about who might be handed the No.6 jersey next weekend

"We release the 21 on Tuesday and then 19 later in the week," Robinson said tongue-in-cheek after being probed whether O'Sullivan might get the nod again.

The 19-year-old was handed the No.7 jersey against the Gold Coast in the 20-12 win in round 18, when regular half Cooper Cronk was on baby-watch duties.

Matterson played five games at five-eighth in his rookie year at the Roosters in 2016 but has played the majority of his 51 NRL games in the back row, with a few at centre as well.

Robinson confirmed to NRL.com that those two players would be high on the list of options.

He had another injury scare in the match when he watched winger Blake Ferguson and fullback James Tedesco crash into each other in the 26th minute trying to collect a Ben Hunt bomb.

"I was thinking of Steve Waugh and Jason Gillespie ... that was what I was thinking," he said, referring to the two cricketers who collided in the 1999 Test series against Sri Lanka. Waugh ended up with a badly broken nose and Gillespie fractured his leg.

Roosters back-rower Ryan Matterson.
Roosters back-rower Ryan Matterson. ©Grant Trouville/NRL Photos

Thankfully, there was no blood or fractures in the Ferguson-Tedesco incident.

"You know when you can see it happening and you're thinking 'Come on, someone is going to pull out here'," Robinson said.

"But you just know that they're not before they even hit. They both got off scot-free and we managed to get the ball back off St George Illawarra.

"So for the car crash that it was, it was probably a good result."