Wales wing George North has suffered a sprain to his posterior cruciate ligament and is likely to miss the Autumn Internationals.
The 25-year-old was injured during Northampton's 57-13 home defeat by Saracens in the European Champions Cup on Sunday.
North was initially hurt during a tackle by Saracens centre Brad Barritt as Northampton were beaten at Franklin's Gardens.
The Welshman attempted to run the injury off after receiving medical treatment but he collapsed shortly after and was replaced by Tom Collins on 29 minutes.
North is expected to be sidelined for a minimum of six weeks, preventing him from featuring in the fixtures against Australia, Georgia, New Zealand and South Africa that begin at the Principality Stadium on November 11.
Wales coach Warren Gatland is due to name his squad for the four Autumn Tests on Tuesday, October 24.
