Redskins linebacker Trent Murphy had to be helped off the field after injuring his knee in the first quarter of Thursday’s preseason game against the Ravens. (Toni L. Sandys/The Washington Post)

BALTIMORE — Outside linebacker Trent Murphy left the visitors locker room at M&T Bank Stadium on crutches and with his left knee heavily wrapped after getting hurt in the first quarter of Thursday’s preseason opener.

Murphy declined interview requests, but Coach Jay Gruden said that the fourth-year linebacker will have an MRI exam Friday. The coach declined to speculate about the severity of the injury.

Murphy got hurt on a third-down stop on the third Baltimore possession. He had to be helped off the field and didn’t return. A second-round pick in the 2014 draft, Murphy is coming off of a career season in which he had nine sacks. He will miss the first four games of the regular season after violating the league’s policy for performance enhancing drugs. Murphy attributed the failed test to a tainted supplement.

Meanwhile, strong safety Su’a Cravens also will have an MRI exam on one of his knees after getting hurt early in the first quarter. It wasn’t clear on which play Cravens injured himself. A person familiar with the situation said the second-year pro suffered either a hyperextension or a meniscus-related injury. Cravens walked out of the locker room with a slight limp and without crutches. He also did not speak to reporters.

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