The injury at best will put him out of action for 10 weeks, effectively ending the season of one of the club's most effective strike players.
In a double blow, would-be replacement Dwayne West was also ruled out until next season after needing further work on a shoulder reconstruction.
Saints boss Ian Millward said: "We hope Paul's injury is not a major snap of the Achilles. If it higher up it is not as major. It will be worse if the injury is lower down.
"I spoke to his wife and she said he seems quite confident.
"If Paul Newlove plays next year I will be one happy person."
In the absence of available and affordable replacements externally, Millward has no hesitation in allocating the centre berth to versatile Kiwi Sean Hoppe.
"Sean played in the centre for us in the Grand Final against Wigan and he will be a good replacement because he is so multi-skilled and reliable. I have no problems in calling on him," Millward said.
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