PITTSBURGH -- Robert Spillane remembers receiving the call that put him on an early flight to Nashville one Friday morning in 2018.

A week earlier, the now Pittsburgh Steelers inside linebacker made the trip to Minnesota for a tryout with the Vikings and returned home to Chicago without a roster spot.

Even as undrafted free agents around the league got invitations to camps, Spillane’s phone and that of his agent were silent in the week that followed the Vikings tryout.

Until the Tennessee Titans called that Thursday night.

The next morning, he was on a plane to a tryout. Soon after, he joined the team for rookie minicamp.

"You’re getting worried, and I tell myself, 'Give me a call, give me a real opportunity,'" Spillane said. "And I'm going to go wherever I have to go and prove that I belong and prove that I will make your team better. And I will help be a good teammate, earn respect for my teammates and my coaches and go from there."

Two years later, he is set to return to Nashville as a starter replacing perhaps the most important player on the Steelers' roster. Spillane will take over for 2019 top-10 pick Devin Bush, who sustained a season-ending ACL tear against the Cleveland Browns on Sunday.

Spillane will be in one of the most important spots on the field as the undefeated Steelers (5-0) look to continue their hot start against impressive running back Derrick Henry and the 5-0 Titans (1 p.m. ET Sunday, CBS).

"We've got a great deal of confidence in Robert," Steelers coach Mike Tomlin said Tuesday. "We will continue to largely go with him in replacement of Devin. We will, in game planning, look at all of our options in terms of dividing the labor up, but Robert will be the primary man responsible for replacing Devin. We're excited about watching him play and play to the standards of our expectations."

Spillane, 24, took over for Bush late in the second quarter on Sunday and continued in his place for the rest of the victory over the Browns, playing 30 snaps. Not only did he immediately fill Bush's spot on the field, Spillane also took over Bush's duties of relaying the defensive playcall as the wearer of the defensive headset.

When asked Tuesday, Tomlin wouldn't say if Spillane will continue those duties against the Titans, only noting that Spillane wasn’t likely to play every snap the way Bush did prior to his injury.

Even so, if Spillane's 30 snaps on Sunday were an audition, he performed admirably and looked like he belonged in the job by registering five tackles and one tackle for loss.

"It's comfortable," Spillane said. "I like speaking to my teammates during [the playcall] and pre-snap. So I've gotten so comfortable with these guys over the past two years. And it really feels awesome because as soon as I stepped on the field, I felt like my teammates trusted me. And they've respected the work that I've put into the past two years.

"So they told me, 'Rob, you belong out here. This is what you're meant to do. So go out here and show everybody what we know you can do,’ and that felt really good to hear from them."

Spillane’s journey to the Steelers' starting defensive lineup began in Nashville, where the Western Michigan product earned a spot on the Titans' 90-man roster with his tryout. An undrafted free agent, Spillane didn't earn a scouting combine invite earlier in the year, despite 312 tackles, 10 sacks, four interceptions and nine passes defended in a four-year college career.

With the Titans, Spillane was a preseason fan favorite, racking up 25 tackles and an interception -- coming against the Steelers -- in four preseason games. He didn't make the 53-man roster, but he did enough to earn a spot on the practice squad. He was elevated from the practice squad in October 2018 but was cut by the end of the month after appearing in just two games.

He spent the rest of the 2018 season without a team before signing with the Steelers in February 2019. The story was nearly the same in Pittsburgh: training camp, practice squad, waived from the practice squad, a return to the practice squad three weeks later and, eventually, an elevation to the active roster in November.

"We like our MAC football," Tomlin said. "We've had our exposure to it, if you will, from a scouting standpoint. [Spillane] was a quality player in college first. I think that's what caught our attention. He was able to get some league experience down in Tennessee. We just had an opportunity to acquire him when he was available.

"We thought he was a guy that could play football, that could add depth and competition to our fold. He's proven that over the course of his stay here. He's continually gotten better and continually worked to carve out a role for himself."

Spillane played 168 snaps on special teams and just one on defense. But when the Steelers parted with Mark Barron during free agency, that single snap made Spillane the third-most experienced inside linebacker on the roster behind Bush and Vince Williams. The others in the room, Ulysees Gilbert III and Marcus Allen, had never played a defensive snap at inside linebacker.

The position had the least depth of any on the team, and Bush, evolving into a playmaker and defensive leader, was the one Pittsburgh could least afford to lose.

But in Spillane, the Steelers have at least a short-term solution to attempt to slow Henry, a player Spillane got a little familiar with during his stint in Tennessee.

"Derrick Henry is a physical specimen who requires 11 men on defense getting to the ball to bring him down," Spillane said. "It's not going to be one thing that's going to help us win the game, but it's going to be a collection of efforts from everyone out on the field."