Rovers said in a statement: “The club wishes Ethan Ryan a speedy recovery. We know he’ll come back stronger in 2022.”
Hull KR also have a serious doubt over captain Shaun Kenny-Dowall for Saturday’s derby against Hull FC after he injured a foot against Wigan.
The former New Zealand international centre has not missed a game since joining Rovers from Newcastle Knights ahead of the 2020 season.
Meanwhile, Hull KR forward Matty Storton has signed a two-year contract extension which will keep him at Hull College Craven Park until the end of 2024.
“I’m looking forward to working with Matty for the next three and a half years.”
Storton said: “I’m over the moon. It was a no-brainer really when I got approached by the club to sign the extension – I snapped their hand off.
“All the coaching staff believe in me. It was an easy decision. I’m learning from a lot of good players at the club and they’re all hitting their primes – so I’m learning from the best really.”
Last week, the Robins also announced that half-back Rowan Milnes signed a three-year extension with the club.
The 21-year-old made his Super League debut last October against Catalans. He has made 10 appearances for the club so far.