Roby, 26, limped off the field in the 17-16 loss at Huddersfield following a heavy tackle, and coach Royce Simmons had feared the worst.
However, the Saints medical team has assessed the problem, and scans have shown only minor damage, meaning Roby is expected to return by the end of the month.
Roby missed just one Super League game last season – the Round 27 win over Harlequins.
He has barely missed a match since become a regular during the 2005 season, scoring 68 tries in 237 appearances for the club.
St Helens host Hull without Roby on Friday, and they also have doubts over Mark Flanagan (thigh) and Tommy Makinson (tendonitus), although Chris Flannery, Josh Perry and Jamie Foster are expected to be back in contention.