The England international sustained the injury in Saints' 52-18 win over Wakefield at Langtree Park last Friday and will see a specialist on Tuesday to discover how serious it is.
"Robes will be out for an extended period of time," said coach Nathan Brown. "We're not sure how long at the moment."
He added: "It is more than likely he will be out for a month and maybe as long as 10 weeks.
"It is definitely something we didn't want to happen but we will move on.
"We will look at the best options and see how we go later in the week."
Without another specialist hooker, Brown looks set to switch New Zealand utility player Lance Hohaia from stand-off to plug the gap.
"I wouldn't want to move our half-backs around but maybe that is the best option," Brown added.
St Helens will have skipper Paul Wellens back for Friday's home game against Salford when he is likely to make the move to loose forward, with Jonny Lomax keeping the full-back role after making a successful switch of positions.
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