BALTIMORE -- The Steelers fear defensive MVP candidate T.J. Watt suffered a Grade 3 left MCL sprain in the third quarter of their pivotal 17-10 win over the Ravens on Saturday, pending further testing, a source told ESPN's Adam Schefter.

Watt suffered the left knee injury shortly after securing his second sack of the afternoon when he landed awkwardly after jumping and colliding with teammate Montravius Adams. Watt, who was ruled out for the rest of the game, wanted to return, but the Steelers wouldn't allow it.

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Watt lay on his stomach, pounding his first into the wet, muddy turf after the play while his teammates took a knee around him. After several minutes in the sideline blue medical tent, Watt, who came off the field under his own power, walked gingerly back to the locker room for further evaluation.

"Obviously, man, T.J. is a big part of not just our defense but our team," linebacker Elandon Roberts said. "Great leader, great guy, on and off the field. I'm hoping it's not as bad. ... From a standpoint of our team -- not just our defense but our team -- we definitely don't want to see T.J. go down."

To that point, Watt recorded two sacks and a tackle for loss Saturday afternoon, bringing his season total to 19. Watt not only leads the NFL with 19 sacks, but he's also the second player in league history to have multiple seasons of at least 19 sacks, joining older brother J.J.

Coach Mike Tomlin was still holding out hope that Watt's injury wouldn't be too serious.

"I'll have more information probably next time we get together," Tomlin said. "Obviously disappointed in that, but there are challenges in the game of football, there are challenges in life. So we'll deal with it, whatever it may be."

With Watt out, the Steelers turned to Markus Golden and rookie Nick Herbig to rotate in opposite Alex Highsmith. Golden, who signed with the Steelers in free agency, sacked Ravens quarterback Tyler Huntley on third down to force Baltimore to settle for a field goal on its final drive.

"I got a lot of sacks hitting the edge and stuff like that, but I just want to get out there and win the game for my team," Golden said. "Get out there and compete for T.J. You got to play hard for a guy like him when he goes down, because that's what he does."

If the Steelers make the playoffs and testing confirms the severe MCL sprain, it's unlikely Watt would be able to play in the first round. That means searching for yet another defensive replacement in a season full of them.

"We better be able to, even though it's a tall ask," defensive captain Cameron Heyward said of replacing Watt. "It's not going to be one guy. We're going to have to mix some things up, but as a pass-rush crew and as a D-line and an outside linebacker crew, we have to step up.

"It goes to guys like Alex Highsmith, who I thought played really well today, Nick Herbig, Markus Golden. These are the guys we relied on for depth all year, and they've done a good job. No one's going to go out there and say, 'Hey, I'm T.J. Watt,' but collectively we got to pick it up."