Jordan Reed catches a deep pass during the fourth quarter against Dallas on Thursday. Reed suffered a shoulder injury in the loss to the Cowboys. (Tom Pennington/Getty Images)

Redskins tight end Jordan Reed, the team’s leading receiver, suffered a Grade 3 AC joint separation in his left shoulder, Coach Jay Gruden said on ESPN980 radio on Friday after the tight end underwent an MRI to determine the severity of his injury.

Reed’s left arm is in a sling, and the team is taking his situation “day by day,” Gruden said. While the coach added that he hoped Reed would be sufficiently healed to play in Sunday’s game at Arizona, he made clear that it was too early to tell that.

In a conference call with reporters earlier Friday, before his radio appearance, Gruden offered few details about the injury, which sent Reed to the locker room for treatment during the second quarter of Thursday’s 31-26 loss at Dallas, but added: “There is something there we have to deal with.”

The 6-foot-2, 246-pound Reed is a crucial element of the Redskins’ passing game, which is the strength of the team as it battles for a spot in the playoffs. Reed has caught a team-high 59 passes for 630 yards and five touchdowns.

He is also among the more determined competitors and returned to the field Thursday afternoon, to the surprise of Gruden and quarterback Kirk Cousins, and caught two of the Redskins’ three fourth-quarter touchdowns. Despite missing a portion of the second quarter, Reed finished with a game-high 10 receptions (on 12 targets) for 95 yards and two touchdowns.

In the locker room after the loss, Reed appeared unable to use his left arm, which hung at his side as he used his right arm to dress himself.

Two starters on the offensive line also suffered injuries. Right guard Brandon Scherff sprained an ankle but returned to finish the game, as did left guard Shawn Lauvao, who sprained a wrist. Right tackle Morgan Moses has been playing on a sprained ankle for the last three weeks.

Reed is the only player who’ll have an MRI, Gruden said.

The Redskins emerged from a punishing stretch in which they played three games in 11 days with 6-4-1 record. Third in the NFC East, they remain in the playoff hunt and must play three of their next four games on the road: Sunday at Arizona, Dec. 11 at Philadelphia and Dec. 24 at Chicago.