Northampton expect their England full-back Ben Foden to be sidelined "for a number of weeks" after he was injured during last Friday's Aviva Premiership victory over Bath.
Foden will see a specialist on Wednesday, but his immediate prospects do not look good.
In a statement, Northampton said: "Ben Foden sustained significant damage to his ankle ligaments during last Friday's Aviva Premiership game against Bath, and he is going to see a specialist tomorrow. The club will have more information after that, but Ben is expected to be out of action for a number of weeks."
Northampton are just three games into their season, having enjoyed an unbeaten start following league victories over Gloucester, Exeter and Bath.
But it now looks as though they will be without 27-year-old Foden for a lengthy period of time.
Saints' Heineken Cup opener against Glasgow is under a month away, while news of Foden's injury will be of considerable concern to England boss Stuart Lancaster.
England begin a tough autumn Test schedule against Fiji at Twickenham on November 10, which is followed by appointments on successive weekends with Australia, South Africa and New Zealand.
Foden would be a key part of Lancaster's plans, but it appears possible the 27 times-capped Saints star could now miss the whole autumn series.
- Ben Foden
- Aviva Premiership