The starters above are listed by the three players who started the most games at a given position. There was no shortage of injury issues throughout the course of the season and every player on that list saw action as the starter at one point, with the exception of the rookie Wheeler.
Brackett stepped into his role as captain of the defense and picked up the slack when Bob Sanders missed time due to injury. He was sorely missed in the playoff contest against the Chargers.
Session proved himself to be a playmaker in 2008
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Senn, an undrafted rookie, played quite well on special teams and when asked to step into the lineup. Davis was signed during the season and stepped into the lineup without much incident, though the Colts would have preferred to have Hagler, Keiaho, and Brackett starting the full regular season and into the postseason.
Indianapolis seems to be able to take pretty much anyone, plug them in at a given linebacker spot, and have that player be productive. A lot of that has to do with the job that Ron Meeks and Mike Murphy do getting the players ready for action every week, but the players involved have an awful lot of say in what happens once the game clock starts.
Session took the next step in his development after a solid rookie season, where he saw some spot duty, again filling in for Hagler. With Brackett returning healthy next season, Keiaho a steady force as well, and Senn and Davis adding quality depth, the Colts are well-positioned for 2009 at linebacker, although Keiaho will be a restricted free agent this offseason.
The Cover 2 defense that Indianapolis runs asks a lot of its linebackers. Principally, it asks them to cover a lot of ground, both in run support, filling the gaps left by the defensive linemen, and in the passing game, filling the zones in the middle of the field.
Although everyone seemed to have a good sense of what their assignment was and where they were supposed to be throughout a season where the unit was riddled with injuries and new players were frequently asked to step up, the linebackers, as a whole, lack athleticism.
With all the ground they need to cover, they need to be faster, have better short area quickness and, in general, be more explosive. Aside from the occasions where Brackett stepped up and made a big play, the game-changing plays on defense belonged to the defensive line and the secondary in 2008.
Disciplined and in position adds up to wins and efficiency in the regular season, but when the playoffs start, someone needs to make a play. With Brackett out of the line-up against San Diego, the Chargers could be fairly certain that none of the linebackers were going to make that play.
It's unlikely that Wheeler will be any less involved in the game plan in 2009 than he was in 2008. With an offseason to learn and a preseason to get his feet wet, Wheeler has the athleticism and skill to be a playmaker at linebacker, both as a situational pass rusher and an every down player.
The Colts will have to make a decision about Keiaho this offseason
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Brackett is signed through next season, Keiaho is a restricted free agent, Senn will be an exclusive rights free agent, and Session has another year of service before he becomes a restricted free agent, so those pieces are in place and aren't going anywhere, with the possible exception of Keiaho.
Davis was signed during the season and will probably test the free agent market. The smart money is on him not finding many suitors, though, so he will end up back in a Colts uniform, at least through the final cutdown before the 2009 season, if not beyond.
Hagler is a different story entirely. He will be an unrestricted free agent, but it is not certain that the Colts want him back for next season. He has struggled through injuries and Session seems as though he is doing a fine job in Hagler's stead.
Chances are good that Indianapolis will let Hagler test the waters, similar to how they handled Josh Thomas during the 2008 offseason, and bring him back only if the price is right, probably for a year, just like Thomas.
Per usual, don't expect the Colts to be big players in free agency at the position, as they prefer young, athletic players that other teams feel are undersized. They like to train and mold these young players into productive moving parts in their system. Look for them to draft a linebacker, though, and bring on at least one undrafted free agent.