Watts, 25, was the Vikings’ only recruit from Down Under in the build up to the campaign, although they have since added Sione Kite and Cameron Phelps to their squad, with the pair hoping to be available for the game at Hull KR on March 4.
The former North Queensland Cowboys and Sydney Roosters half-back or hooker was making his Super League debut against Salford on Sunday, but picked up the injury in a tackle, and scans have since shown the dreaded ACL injury.
It is a second ACL injury blow for Watts in the space of 18 months – he suffered the same injury to his left knee in pre-season training with the Roosters when holding a tackling bag for back-rower Anthony Cherrington, his knee buckling awkwardly beneath the impact.
Watts tweeted: “Not good news, scan shows acl gone. Out for the season,” and later added: “Thanks so much for all the kind messages, I will be back better than ever! Now focused on helping the boys and the club moving forward.”
It is a further injury blow for the Vikings, who have also lost hooker Scott Moore (broken jaw), long-serving forward Ben Kavanagh (Achilles) and Thomas Coyle (broken leg) in recent weeks, while new signing Shaun Briscoe is yet to make his competitive debut for the club because of injury.