Fairley sprained his right MCL and PCL, NFL Media Insider Ian Rapoport reports, via a source informed of the diagnosis.
The hope is that Fairley will be back in a month, per Rapoport, but there is no firm timetable for his return. He might need more tests before that happens.
Barring a setback, Fairley should return in time to put together a few strong games for a contract run. He had been enjoying the best season of his career prior to Sunday's injury.