Ireland head coach Declan Kidney originally named Sexton in his side for the clash with Italy on Saturday but on the same day, Sexton went over on his foot in training, damaging the tendon.
Four to six weeks would mean Sexton would hopefully be back for the business end of Leinster's RaboDirect PRO12 run in - but Ireland coach Anthony Foley believes the 27-year-old could well be back in action sooner.
"We were told yesterday that Jonny will be out for four six to weeks," said Foley.
"That was a doctor saying four to six weeks, it wouldn't surprise me if Jonny is two to four weeks away.
"You take into account his powers of recovery from his hamstring tear and how quickly he came back from that, combined with his attitude to rehab, and he could be back sooner.
"Having worked with the lad this season, I imagine when he gets into the thick of the rehab he'll fly through it, although it will be tough mentally at the start."
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