Trent Murphy is helped off the field by the training staff Thursday night. (Photo by Toni L. Sandys/The Washington Post)

It took 27 plays of preseason football before the Washington Redskins’ depth at outside linebacker was abruptly challenged. Outside linebacker Trent Murphy will miss the rest of the season because of a torn anterior cruciate ligament in his left knee, according to a person with knowledge of the situation.

An MRI exam Friday revealed the injury, which happened during the Redskins’ third defensive series of Thursday night’s preseason opener against the Baltimore Ravens at M&T Bank Stadium. Murphy didn’t talk to reporters after the game, but he left the visitors’ locker room on crutches with his left knee heavily wrapped.

The situation leaves Washington with one less quality option at the position, which has had a notable injury for three straight years; Junior Galette missed the past two seasons because of torn Achilles’ tendons. Murphy finished second on the team with nine sacks last year, taking a significant step in his development as a pass rusher that continued this summer during training camp before the injury.

Murphy, 26, was slated to miss the first four games of the season for violating the NFL’s policy on performance-enhancement drugs. He will serve the suspension, despite the injury, forfeiting four game checks totaling $185,315 of his $787,591 base salary in 2017.

The injury also will have financial ramifications for Murphy as a pending free agent next offseason. The second-round pick in 2014 was in the final year of his rookie deal and won’t have the opportunity to prove his value on the field in a contract year.

The Redskins had enough depth at outside linebacker to navigate around Murphy’s suspension with Ryan Kerrigan, Preston Smith, Galette and Ryan Anderson. Now, they’ll have to hope they have enough talent to handle an entire 16-game season without Murphy, but many questions remain after the first preseason game.

Smith missed Thursday’s contest because of a sprained ankle he suffered last week in practice. Redskins Coach Jay Gruden said he could be sidelined for “a week or two” at the time of the injury. Galette wasn’t in uniform because of a hamstring injury, although he participated during practice leading up to the Ravens’ game.

Beyond the training camp injury concerns, the Redskins lacked a consistent pass rusher last season to pair with Kerrigan, who earned Pro Bowl honors with a team-high 11 sacks. Smith, a second-round pick in 2015, displayed flashes of his potential but was inconsistent. He finished with 4.5 sacks in his second year after recording eight as a rookie. Smith, 24, looked stronger during training camp before the injury, but it’s unclear how that will translate in live action.

Galette has been one of the most impressive players during training camp, displaying the skill-set that produced 22 sacks in two seasons for the New Orleans Saints in 2013 and 2014. But Galette has yet to be in uniform for a game as a Redskin because of two season-ending injuries. He hasn’t appeared in an NFL regular-season game since Dec. 28, 2014.

Anderson now becomes defensive coordinator Greg Manusky’s fourth option in the outside linebacker rotation. Anderson is a physical player with good energy, recording three combined tackles against Baltimore, but he is still getting acclimated to the league after his first NFL game.

“It was something to build off of,” Anderson said. “I know what to work on, and I got a good feel for the speed of the game. I was just trying to play with intensity. I need to watch the film and then build off of this.”

More Redskins coverage:

Five observations from Washington’s preseason-opening loss

In Redskins’ disappointing preseason debut, few moments for optimism

Kirk Cousins sings Whitney Houston, completes one pass in preseason debut

Scot McCloughan just did a Twitter Q&A, and Redskins fans will love his responses