The 30-year-old flanker suffered the injury just eight minutes into Llanelli's European Cup defeat to Toulouse on Saturday and was immediately taken to hospital for an operation.
Easterby, the Scarlets captain, will definitely miss next weekend's European clash with Edinburgh and Llanelli director of rugby Gareth Jenkins does not expect him to be fit for Ireland's November appointments with New Zealand, Australia and Romania either.
"It was a nasty break and when he came off it was clear that Simon was in a really bad way," said Jenkins.
"It is a great shame because he is a player who has hit real form. He will be missed by us and by Ireland in their autumn internationals."
Munster flanker Alan Quinlan is also a doubt for Ireland after injuring his knee in the second half of Friday's European Cup defeat to Sale.
Quinlan, who had an initial scan on the injury in Cork on Saturday, was due to have a scope put in his knee on Monday to pinpoint the extent of the injury.