Hull KR coach Tony Smith says vice-captain Elliot Minchella suffered a suspected ACL injury in the defeat at St Helens.
Minchella, 25, has been in impressive form in the opening two games of the season, making 46 tackles after coming on off the bench as the Robins went down 25-0 to the champions.
But he went down late in the game and was helped off the field, and in his post-match press conference, Smith said they feared the worst.
Smith said: “I’ll keep my fingers crossed for him overnight and hopefully we’ll get some better news in the next day or two.”
They had earlier lost prop Korbin Sims, who was wearing a Masoe 10 shirt in honour of former Robins favourite Mose Masoe, after just three minutes due to a head injury and later lost Matty Storton to a head injury too.