Linebacker Sean Weatherspoon, dubbed the “Baby Lion” by Mike Peterson when he was a rookie, grew up during his second season.

The 2010 first-round draft pick settled in nicely at weakside linebacker with 125 tackles, 14 tackles for losses, eight passes defensed and four sacks.

“I think Sean had a breakout season this year,” Falcons coach Mike Smith said. “People took notice in terms of his ability around the [NFL]. You saw that in the Pro Bowl voting as well. He garnered some attention there. I think he’s going to be a great player for us in the future.”

Curtis Lofton, the team’s leading tackler with 159, is a free agent along with defensive end John Abraham, the leader in sacks with 9.5.

Cornerback Brent Grimes, safety Thomas DeCoud and defensive end Kroy Biermann are also free agents on a defense that finished 12th in the league, giving up 333.6 yards per game.

The pass defense improved slightly from ranking 22nd in the league in 2010 to 20th in 2011.

The unit struggled mightily on third downs. They finished the season ranked 28th in the league in third-down efficiency (93 of 211 for 44 percent).

“Obviously, there are still areas that we can improve upon,” general manager Thomas Dimitroff said. “We need to continue to pressure the quarterback. We need to continue to get our hands on balls and capitalize on interceptions. With that said, I love the tenacity of this defense.”

They finished sixth against the run (97 yards per game and believe they are not far away from a breakthrough.

“We just have to minimize the mistakes and take advantage of all of the opportunities that we do have when we are presented with them we have to make the most of them,” defensive tackle Jonathan Babineaux said. “We can’t fall short.

A look at the Falcons’ defense by positional grouping:

Defensive linemen

The players: Abraham (free agent), Babineaux, Biermann, Corey Peters, Ray Edwards, Lawrence Sidbury, Cliff Matthews, Peria Jerry and Vance Walker (restricted free agent).

The skinny: Abraham suffered a groin injury in the second game of the season, and the Falcons kept him on a play-count until the playoffs.

The tandem of Abraham and Edwards never materialized as Edwards struggled after knee surgery and with fitting into the new scheme. He finished with 3.5 sacks, but played the run well, according to Dimitroff.

Abraham is a free agent. If he returns, he would be the league’s leading active sacker, with 112.

Sidbury was active in a limited role and registered four sacks. Why he hasn’t seen more action is one of the great mysteries around Flowery Branch.

In six seasons with the Falcons, Abraham has 58.5 sacks.

“I’m thinking that they will bring Abe back,” Babineaux said. “I think that’s what they will do. But we all know it’s a business at the end of the day, and we hope they’ll make the best decision for this team.”

Linebackers

The players: Weatherspoon, Lofton (free agent), Peterson (Injured reserve, free agent), Stephen Nicholas, Spencer Adkins, Akeem Dent, Robert James.

The skinny: Weatherspoon and Lofton turned in fine seasons. Nicholas was slowed by quadriceps and toe injuries.

Peterson suffered a torn triceps late, but played well in reserve.

Adkins held his own while getting some action down the stretch, and rookie Akeem Dent had 18 special-teams tackles.

The unit was clearly the strength of the defense.

Defensive backs

The players: DeCoud (free agent), Grimes (free agent), Dunta Robinson, William Moore, Shann Schillinger, Darrin Walls, Christopher Owens, Dominique Franks, James Sanders and Kelvin Hayden.

The skinny: The defensive backs played better in coverage and would have benefitted from a better pass rush. The group battled down 50 passes and had 14 interceptions.

Grimes, after going to the Pro Bowl as an alternate in 2010, missed five games after what the team called a “minor procedure.” The Falcons will have a tough free-agency call on him.

Franks played well as the nickel back in the slot and outside at left corner for Grimes. He could be a keeper.

Robinson provided a physical presence, but was inconsistent in coverage.

Moore is a big hitter, but still gets lost in coverage.

Special teams

The players: Matt Bosher, Matt Bryant, Joe Zelenka and Eric Weems.

The skinny: Bosher settled down after struggling early in the season. He finished with a 42.7 average and placed 27 punts inside of the 20. Bryant made 27 of 29 field goals. His two misses were from 40 to 49 yards.