Jon Gruden said earlier this week that Marshawn Lynch's groin injury was "cause for concern," and an MRI apparently has proven him right.
Lynch is expected to miss at least a month with the injury, NFL Media reported Friday, adding that the issue could be enough to land the running back on injured reserve.
The Raiders have a bye this week; Gruden said Tuesday he would have an update on Lynch's prognosis next week.
Lynch carried 13 times for 45 yards in Oakland's 27-3 loss to the Seahawks in London on Sunday. His last carry came with about 10 minutes left in the game when the Raiders were trailing 27-0.
Doug Martin and Jalen Richard should be next in line to replace Lynch, but Martin didn't get a carry Sunday and Richard had just two.