The St. Louis Rams' 35-11 Thursday night loss to the San Francisco 49ers was a costly one.
Rookie strong safety T.J. McDonald was fitted for an air cast and carted off the field after getting his leg caught in traffic in the third quarter.
Two sources informed of the injury told NFL Media Insider Ian Rapoport that McDonald suffered a leg fracture on the play.
The third-round draft pick was a Day 1 starter for the Rams. His replacement, Matt Giordano, immediately was targeted in coverage on Vernon Davis' 12-yard touchdown.
Veteran nickelback Cortland Finnegan also exited with a thigh injury. Having lost a step from last season, the 29-year-old has been among the least effective cornerbacks in the league through four games.
UPDATE: The Rams placed McDonald on their injured reserve designated to return list Friday.
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