Courtney Lawes will see a specialist next week to determine the extent of an ankle injury.
The England international suffered the problem during Northampton's 34-14 defeat by Sale on Tuesday night.
Lawes departed in the ninth minute after falling awkwardly when challenging Tom Curry for a high ball, the second row catching his right ankle in the turf as his body twisted on landing.
A Northampton statement read: "Northampton Saints can confirm that Courtney Lawes sustained an ankle injury in last night's defeat to Sale Sharks which requires further examination by a specialist next week, when the timeframe for his recovery will become clearer."
The news is a setback for England coach Eddie Jones, who is due to name an initial training squad on Monday ahead of five Tests following a non-cap encounter with the Barbarians.
That fixture, at Twickenham on October 25, will be England's first outing since beating Wales at home in March.
