ASHBURN, Va. -- Washington Redskins linebacker Reuben Foster suffered a season-ending tear of the anterior cruciate ligament in his left knee Monday, a source told ESPN's Adam Schefter.

He was injured on the third snap of the team's first organized team activity session and was carted off the field. He later underwent an MRI. A source said that Foster will see a specialist to see if his knee suffered more damage than just the ACL tear.

Redskins coach Jay Gruden said Foster stepped on the foot of offensive lineman Tyler Catalina as he rushed a gap at three-quarters speed.

When Foster was on the ground, he was surrounded by multiple team officials, including president Bruce Allen and Gruden. Former Alabama teammate Landon Collins also joined them and Malcolm Bracken, the director of player development, kneeled and patted Foster on the chest. At one point Foster turned to his right and pounded the turf with his fists. He could be heard crying, putting his hands to his helmet as his head bobbed. As he was carted off, Allen ran back to the facility to be with Foster.

It was Foster's first rep in full-team work with Washington, and it occurred during a session Gruden called a "jog-through."

It's the latest injury for a team besieged by them the past two years. The Redskins have had a combined 52 players on injured reserve the past two seasons, though a handful of them were hurt in camp and might not have made the roster. Last season they lost key players such as quarterback Alex Smith, running backs Derrius Guice and Chris Thompson, and tight end Jordan Reed. The Redskins have gone 7-9 each of those seasons.

"I don't know how to process it," Gruden said. "We've had some bad luck over here for the last couple years, but this one here takes the cake because this was a non-contract drill and there was no contact involved in it. He just landed funny. But we have to move on. We have to get guys ready to play that are here. We've got to do the best we can to get Reuben healthy again and move on.

"Sometimes I wish we'd do just all walk-throughs and then go play on Sundays, but we've got to practice."

Gruden was in favor of drafting Foster two years ago, but the Redskins opted for Jonathan Allen instead. But Gruden always liked what Foster could add. The Redskins cut starting linebacker Zach Brown earlier this offseason, but they likely would have done that even if Foster wasn't around.

"I love the energy he brings to this team," Gruden said of Foster. "... I was excited to see him, for sure. He's devastated; he felt something happen in his leg. We've got to get all the tests back, but he's very upset about it. He was pretty hurt."

The Redskins claimed Foster off waivers from San Francisco in November, three days after he was arrested for a domestic violence charge. The charges were later dropped and the NFL decided last month not to suspend Foster.

The Redskins had planned to start him as one of their two inside linebackers, probably alongside former Alabama teammate Shaun Dion Hamilton. Washington loves Foster's speed and athleticism inside.

It's uncertain how the Redskins will replace Foster. Former starter Mason Foster, often criticized for his lack of speed, said he lost 15 pounds this offseason in an attempt to get faster. The Redskins drafted a linebacker, Cole Holcomb, in the fifth round last month and they also have veteran Josh Harvey-Clemons, considered more of a nickel linebacker. They could look to add a veteran.

Foster suffered an ankle injury as a rookie with the 49ers, causing him to miss five games. He was suspended for the first two weeks of 2018 for a weapons offense and a misdemeanor drug offense. Another domestic violence charge, involving the same woman from November, was dropped.