Saints star George North has been ruled out for between six and eight weeks after the extent of the injury he suffered against Saracens last Sunday became clear.
North was forced off 27 minutes into the Champions Cup clash at Franklin's Gardens, having initially tried to play on after being involved in a collision while carrying the ball.
He went to ground after running down the right wing unopposed and there was anxiety in the stands as the 25-year-old had to be helped from the field.
North was sent for a scan on Monday and Saints have now announced that he has suffered a sprain to his posterior cruciate ligament.
That means North will definitely miss Saturday's Champions Cup trip to Clermont Auvergne.
He will also be sidelined for the home clash with Wasps a week later and is out of Wales' autumn international campaign, which begins on November 11 and ends on December 2.
Saints will hope for a speedy recovery, with a Champions Cup double-header against Ospreys as well as Premiership games against Exeter Chiefs and Harlequins to come between mid to late December.