Kupp suffered a right high-ankle sprain in Sunday's 27-17 loss to Arizona, and the prognosis "doesn't sound good," Adam Schefter of ESPN.com reports.
The Rams had some optimism around Kupp's injury after initial tests revealed that he avoided a fractured ankle, but further examination revealed ligament damage. While a return timetable for the star wide receiver has yet to be established, high-ankle sprains typically take 4-to-6 weeks to recover from, and the Rams can ill afford to lose Kupp for that long considering they're on the verge of falling out of the playoff race at 3-6.