If you were to ask me who I think the most underrated player on the Indianapolis Colts' roster is, I'd probably answer that its left tackle Anthony Castonzo.
Castonzo has been very good through his first four years in the league, and the best news is that he has continually improved. Drafted in the first round of the 2011 NFL Draft by the Colts out of Boston College, Castonzo was drafted to protect quarterback Peyton Manning's blindside. In reality, however, the lineman never played a single snap with the legendary quarterback, who missed the entire 2011 season with a serious neck injury. In 2012, the Colts drafted Andrew Luck, and ever since then, Castonzo's role has been to protect the blindside of one of the league's brightest rising stars. In that role, Castonzo has done a very good job - and that starts with his availability. Including playoffs, the lineman has started 62 games in a row (66 total) and, in 2014, he played more snaps than any other offensive lineman. He does a solid job in run blocking but has really developed into a great pass protector, as in 2014 he allowed just two sacks and nine hits in 726 snaps (according to Pro Football Focus). He was named to Pro Football Focus' ranking of the top 101 players of 2014 (at number 66), and he was also named to NFL.com's All-Underrated Team this offseason.
When it comes to his importance to the Colts, Castonzo's value is expressed in two main ways. Firstly, he's simply a good player - and it normally follows that the better the player, the more important they are (this isn't always the case, but it's a general principle). He does a good job all-around but really impressed as a pass blocker in 2014, keeping Andrew Luck very clean considering the amount of times the quarterback dropped back to throw. Secondly, however, he's crucial to the offensive line. Just take a look at the players surrounding Castonzo at left tackle. Behind him on the depth chart are project players Ulrick John and Denzelle Good, and there's simply no good option to replace Castonzo if he were to go down. None at all. Beside him on the starting offensive line are a lot of question marks. Jack Mewhort, who played at left guard last year and did a good job, is moving to right tackle. Right guard seems to be ok, with veteran Todd Herremans filling the role, but the center and left guard positions are big question marks right now as we approach training camp. Castonzo won't have great play surrounding him on the offensive line, just like there's no good option whatsoever to replace him if he were to be injured.
Both of those things, coupled with the reality that the Colts are a passing offense and that the team's entire success rides on Andrew Luck, whom Castonzo is tasked with protecting, lead to the left tackle being an incredibly valuable piece for the team in 2015. He is a player that the Colts absolutely can't afford to lose, and he comes in at number three on our list of the most important players for Indianapolis in 2015.
Past rankings on this list: 13th (2014); 4th (2013)
Check out our list counting down the 15 most important players to the Colts' success in 2015:
15. Jerrell Freeman, ILB
14. Greg Toler, CB
7. Mike Adams, S
3. Anthony Castonzo, LT