Roby, 27, is thought to have suffered a similar injury to the one that kept him out for 10 weeks earlier this season.
He limped off during Saints’ 40-24 win over Castleford on Sunday and the club’s physios believe he may have damaged his ankle ligaments.
Coach Nathan Brown said, “They (physios) feel he may have done the exact same injury on the other foot. If it is that same injury, then it’s 10 weeks out.”
A 10 week lay-off would mean Roby will be be available for Saints if they make the play-offs.