PITTSBURGH -- With wins Sunday, the Pittsburgh Steelers and Tennessee Titans are barreling toward a Week 7 meeting of the unbeatens in Nashville. But the Steelers will enter the matchup down one of their most important players.

Inside linebacker Devin Bush, the Steelers' 2019 first-round pick, is done for the season after tearing his ACL in the second quarter of Sunday's win against the Cleveland Browns, sources told ESPN's Adam Schefter.

The blow to the defense is incalculable. Bush played every single defensive snap before his injury, and he wears the green dot on his helmet during games to communicate with the sideline as the defensive playcaller.

"It just makes you sick when someone, a young player, skilled like that, gets hurt," Steelers quarterback Ben Roethlisberger said after the game. "I need to send my thoughts and prayers to him."

Before he went down Sunday, Bush recorded half a sack, bringing down Browns quarterback Baker Mayfield with the help of Stephon Tuitt. He also had one QB hit and five total tackles.

Statistically, Bush was off to a quiet start with just 26 total tackles, two pass breakups and one sack through five games. In a solid rookie season a year ago, Bush finished with 109 tackles, one sack, one forced fumble, four fumble recoveries and two interceptions.

Depth at inside linebacker was a major concern entering the season, and Bush's season-ending injury is one of the Steelers' worst fears realized. Veteran Vince Williams typically plays alongside Bush, excelling at run protection, while Bush often drops into coverage -- an area of his game he felt was improving this season.

"I feel like pass coverage has improved drastically from last year," Bush said a few days before Sunday's game. "Still some things I have to work on, but as a whole, I think I'm in a good spot."

When Bush went out with just more than 2:30 left in the first half, the Steelers turned to third-year pro Robert Spillane, a special teams mainstay who was elevated from the practice squad a year ago. With Spillane in, Williams shifted his focus to coverage, while Spillane primarily honed in on run-stopping and blitzing duties, finishing with five total tackles and one tackle for loss.

"Spillane more than answered the bell, but we have a great deal of confidence in him," coach Mike Tomlin said Sunday. "He's a capable guy. Guys that make plays in special teams, that's usually a precursor to them making plays in defense. Since we brought him up the middle of last year he's been a consistent performer for us. I don't think any of us are surprised from quality play from him."

Even though Spillane played well Sunday, Derrick Henry and his 117.6 rushing yards per game average on the horizon makes Bush's sudden loss and the subsequent shuffling of inside linebacker responsibilities especially bad timing for the Steelers.

"Devin is a great player, high-effort guy," outside linebacker Bud Dupree said. "He's one of the most athletic, fastest guys on the team. Anytime you lose somebody like that, it's a big blow."

Before this season Spillane, who initially signed with the Titans as an undrafted free agent in 2018, played only one defensive snap.

Beyond Spillane, the Steelers have two options for inside linebacker on the roster, though neither has much experience. Ulysses Gilbert, a 2019 sixth-round pick, has been inactive for every game this season. He was active for seven games last season, primarily playing on special teams. He finished the year on injured reserve with a back injury.

The Steelers also could turn to Marcus Allen, a safety who started taking reps with the inside linebackers during training camp to prepare for a hybrid role. A physical player with a 6-foot-2, 215-pound frame, Allen has been active for four of five games this season and is primarily a special teams player.

Absorbing Bush's responsibilities might not come down to one player. It could lead to more substitutions and rotations to handle everything Bush did for the team.

"If Devin does have to be down for a while, those [players] will be called to step up," Tomlin said of his trio of backup inside linebackers. "[Spillane] will be among them, Gilbert will be among them, Marcus Allen will be among them.

"We'll put together a formula that allows us to function in the interim, believe it."