The retiring Lewis will replace Paul Gallen as Sharks skipper in the preliminary final against Melbourne, after the Cronulla lock was ruled out on late Thursday evening with a shoulder injury.

The Sharks remain hopeful he will be fit to play in next Sunday's grand final, but Cronulla must first topple the Storm in Melbourne to qualify.

Fellow Sharks co-captain Wade Graham is also unavailable, having been ruled out for the season with a knee injury suffered in the first week of the finals.

Lewis has captained Cronulla once before, but only while acting as a co-captain alongside John Morris and Jeff Robson in round 26 against Canberra in 2013.

He would also likely have taken the role earlier this year, but was injured alongside both Gallen and Graham when Andrew Fifita assumed the captaincy.

"He's been part of our leadership structure for a long time," Flanagan said of Lewis.

"The amount of games he's played and what he does for the game and away from the game as well. It's no surprise he is captain."

"Friday night's game at AAMI Park will represent the end of an era for either Lewis or Melbourne's fellow 300-game veteran Billy Slater, who have 46 Tests and 48 State of Origins between them but will both retire at the completion of the season.

Flanagan also revealed the popular member of the Sharks' squad had been close to taking the role on a full-time basis far earlier in his time at the club.

"We were going to make him captain a couple of years ago and he didn't want it," Flanagan said.

"Gal was thinking about standing down. We had a three-tier leadership group with Michael Ennis, Wade and Gal.

"We wanted Lewi to be part of that and he didn't want to. He wanted to focus on the tail-end of his career and playing football.

"I respect that. But when I told him he was captain (against Melbourne) he was really chuffed about it and looking forward to it. He doesn't want it to be his last game."

Australian Associated Press