Last season’s Man of Steel sustained the injury in Friday’s engage Super League defeat by Wakefield and will now miss the Challenge Cup semi-final against Huddersfield on August 9.
The 23-year-old adds to coach Mick Potter’s selection problems with Sean Long, Jason Cayless and Maurie Fa’asavalu already sidelined.
And Lohnny Lomax could be out for four to six weeks if a scan reveals ankle damage.
Saints were at least boosted by the return of veteran hooker Keiron Cunningham on Friday and Francis Meli’s hamstring injury is not thought serious.
