Steelers' position outlook: Defensive line

PITTSBURGH -- This is the next in a series that takes a post-free agency, post-draft look at all of the positions with the exception of quarterback.

Our fifth look is at a unit that is in transition.

Defensive line

Who is new: Cam Thomas, Stephon Tuitt and Daniel McCullers. The Pittsburgh Steelers signed Thomas to a two-year, $4 million contract in March, and the former Charger, also known as "Baby Zilla," can play nose tackle and defensive end. The Steelers took Tuitt, a defensive end, in the second round of the draft. They used the second of their two sixth-round selections on McCullers, a 6-7, 352-pound defensive tackle.

Who is gone: Brett Keisel, Ziggy Hood and Al Woods. Keisel, who has started at defensive end for the Steelers since 2006, remains unsigned. Hood and Woods signed with the Jaguars and Titans, respectively, as unrestricted free agents.

Returning starters: Cameron Heyward and Steve McLendon. Heyward played sparingly his first two seasons in Pittsburgh but he showed in 2013 why the Steelers could no longer keep him off the field. Heyward supplanted Hood as a starter five games into last season, and the 2011 first-round pick led the Steelers with 31 quarterback pressures and tied for second on the team with five sacks. McLendon took over for five-time Pro Bowler Casey Hampton at nose tackle and probably received too much blame for the Steelers yielding 4.3 yards per carry last season. McLendon has received several votes of confidence from the Steelers this offseason, and he should be better in his second season as a starter. "I'm not worried about him at this time," defensive line coach John Mitchell said.

Most significant addition: Tuitt. The former Notre Dame star already has an NFL body and the 6-5, 303-pounder should play immediately given the Steelers' need at defensive end. The Steelers were elated that Tuitt lasted until their pick in the second round of the draft. They will be even happier if he develops quickly enough to start sooner rather than later opposite Heyward.

Most significant loss: Keisel. The unquestioned leader of the defensive line and a Steelers captain, Keisel finished third on the team with 26 quarterback pressures last season and also recorded four sacks despite missing four-plus games because of a nagging foot injury. The Steelers haven't shown any inclination to bringing Keisel back for a 13th season, which is curious considering how thin they are at defensive end.

On outside looking in: McCullers. The Tennessee product is probably a project more than a player who can provide immediate help up front. McCullers will have to prove to the Steelers he can play with the necessary pad level at nose tackle, and that will be a challenge given his sheer size.

Hidden number: McLendon had nine quarterback pressures last season -- only two less than outside linebacker Jarvis Jones -- despite playing only 338 snaps and coming off the field on passing downs.

Outlook: A defensive line that had been anchored for years by Keisel, Hampton and Aaron Smith has gotten very young. Heyward is the leader of the unit, and he only turned 25 a couple of weeks ago. The biggest question is who will start opposite Heyward. Right now it looks like a rotation of Thomas, Tuitt and Brian Arnfelt or Nick Williams will be used at right defensive end. The Steelers should also keep Keisel on call -- if he doesn't sign elsewhere -- in case some of the young players at defensive end don't progress quickly enough.