NASHVILLE, Tenn. -- The best piece of advice that linebacker Colin Allred has received in the past couple days came from the man he's replacing.

The second-year Tennessee Titans linebacker still doesn't have his own football card. So Keith Bulluck, the 10-year NFL veteran whose season is over because of a torn left anterior cruciate ligament, told Allred to earn that card by playing a great game in his place.

"I'm really excited, but it's bittersweet to honest with you," Allred said Wednesday. "You don't want to see Keith Bulluck go down, especially you don't want to see the severity of the injury. It's like replacing your big brother or your mentor or anybody who has a job can say somebody who showed them the ropes and one day you have to walk in for them. It's kind of tough. I'm excited about it. I'm going to give it everything I've got."

The last time the Titans were without two defensive starters because of injuries, the result was a 59-0 snowy beatdown in New England in October. Now Tennessee is without Bulluck, its leading tackler, and linebacker David Thornton before Friday night's critical game with the San Diego Chargers (11-3).

Bulluck remains on the Titans' roster for now as he awaits surgery on his left anterior cruciate ligament. Thornton had surgery on his shoulder Tuesday and is on injured reserve. Bulluck is the team's leading tackler with 118, and he also had three interceptions.

Allred is taking Bulluck's spot in what will be the first NFL start of his career. Rookie Gerald McRath will make his fourth start, and middle linebacker Stephen Tulloch, Tennessee's second-leading tackler, will put the defense in the right spots.

"Tully's doing fine, and so is Colin and Gerald, too," Titans coach Jeff Fisher said. "They know what to do, they understand the challenges, they understand the potential problems with the matchups. So they'll be fine."

Allred technically is a second-year pro, but he has been with the Titans since 2007 when he spent the first 13 games on the practice squad. A graduate of Baylor, he has deferred enrolling in law school at California-Berkeley while he pursues football. Fisher said the 6-foot-1, 238-pound Allred knows all three linebacker positions.

As Allred talked to reporters at his locker, center Kevin Mawae walked by and yelled out, "Sixth man up, sixth man up."

"If you look at successful teams like the Colts, they have that kind of next-man-up mentality," Allred said. "That's what we're trying to develop here. I know what I'm doing. I've been here, so I feel like it will go well."

McRath had plenty of starting experience at Southern Mississippi before coming out of school early, and he ran the fastest 40-yard dash at the NFL Combine with a time of 4.49 seconds. He has 24 tackles in his limited play as a rookie, but as he found himself the target of reporters, a team official tapped him on the shoulder to turn around for the TV cameras.

Safety Michael Griffin believes the Titans are better positioned to move on now than in October when the secondary was without cornerbacks Cortland Finnegan (hamstring) and Nick Harper (broken right forearm) and nickel back Vincent Fuller (broken right forearm). That forced rookies Ryan Mouton and Jason McCourty to start in New England.

Going into Week 16? No rookie jitters with McGrath and Allred, and Griffin said Tulloch is ready as well.

"He was overshadowed by David Thornton and Keith Bulluck," Griffin said. "As a whole, we had three great linebackers, and right now we have one great linebacker. But we have younger guys who have the capability of becoming great linebackers in this league."

The Titans could be without another defensive starter. Harper missed a second consecutive practice because of a sore shoulder, and Rod Hood worked in his place. Hood had three interceptions while Harper was out, but he hasn't played in five consecutive games. Fisher said he expects Harper to be ready.

Notes: DT Tony Brown (knee/quadriceps muscle) also missed a second practice but told reporters he will play. "This is one they'll have to pull me out," he said. "I'm planning on going." ... DE Kyle Vanden Bosch (neck) also sat out a second day of practice, but he's expected to play.