Sydney Roosters five-eighth Luke Keary will miss a re-match with his old club South Sydney after scans revealed the medial ligament damage in his left knee will keep him out for up to a month.

That means the lively No.6, who is a vital part of the Roosters attacking make-up having dished out 17 try assists and scoring five himself so far this year, will have just one game on the park (Eels) before the Telstra Premiership finals begin on September 7.

He will miss Saturday's match against the Cowboys at Allianz Stadium, the following weekend's highly-anticipated clash with the Rabbitohs and matches against the Raiders and Broncos in rounds 23 and 24.

The Roosters lost to Souths 26-14 in round six this year and were eager to dazzle their arch-rivals with a vastly improved performance this time around.

Match Highlights: Roosters v Dragons - Round 20, 2018

But Keary won't be a part of it, leaving coach Trent Robinson to choose between experienced campaigner Ryan Matterson or rookie Sean O'Sullivan.

Matterson told NRL.com on Monday he would answer any No.6 call Robinson put to him.

"I played there in 2016," Matterson said after a recovery session at Moore Park after the strong 36-18 win over the Dragons on Sunday.

"I just enjoy playing football and I want to be anywhere on the field they want me. I will do my best no matter what the position."

Matterson wore the No.6 jersey five times in 2016. But in the past two years, he's mostly been at second rower or occasionally in the centres.

"It would be different to what I've been used to, but I'll be sweet," he told NRL.com.

"In under-20s for NSW and Australia I played in the halves. But I was 10 kilos lighter then.

"I might be the tallest five-eighth around ... then again [Rabbitohs] John Sutton was pretty big at five-eighth."