When looking at the most important players on a team, you can look at it in one of two main ways. One, you could look at how good the player is overall, or two, you could look at how the talent around him or behind him looks.
Both of those came into consideration for all of these players, although more of them probably leaned towards the best player side rather than the necessity side. Perhaps the best example is tight end Coby Fleener, who made the list at number 12. He's not even the team's number one tight end, so in his case it's definitely not the need that was the predominant factor. Instead, it was the expectations of him and his potential in the passing game, along with the way that the offense will incorporate him this season.
Others, like rookie Bjoern Werner (number 8) appeared on this list mainly because the team doesn't have another pass rushing option to complement Robert Mathis. While Werner didn't have an overly impressive camp or preseason, he appears on this list nonetheless because of how important it is to the team that he plays well.
Left tackle Anthony Castonzo comes in at number 4 for largely the same reason.
First off, consider the players behind him. We got to see that first hand in the third preseason game, where on the third play of the game Castonzo got his leg rolled up on and went down. He initially thought it was season ending and when I saw it happen, I thought I was going to have to write this about a player not yet ready to return from injury. Fortunately for the Colts, however, he is expected to be ready week one for the Raiders, and it's a really good thing especially after we saw his backup play.
Joe Reitz filled in and played pretty badly in the third game against the Browns, highlighting how important Castonzo is. If he were to miss regular season time, Jeff Linkenbach would be an option there as well, and while he's an upgrade over Reitz along the line, he's not a great option.
Secondly, consider the talent starting alongside Castonzo on the line. Donald Thomas at left guard is looking like the team's second best lineman (behind Castonzo), and from there on it hasn't looked too good at all. Samson Satele at center sucks, that's been clearly established on this site and on most others too. At right guard is a player much worse than Satele, and that's been established even more so than the fact that Satele sucks: Mike McGlynn really sucks. And lastly, one of the highest paid right tackles in the NFL, offseason addition Gosder Cherius, was a very good pass protector for the Detroit Lions but hasn't had a great preseason, especially in the third game, where he was completely embarrassed by Paul Kruger all night.
So yeah, that doesn't sound like a great offensive line, and a Colts team that generally runs to the left more anyways (which is uncommon) will likely do the same again this year. And a quarterback who excels at avoiding pressure will likely have to do a lot more of that this year.
What the offense needs to be able to count on is that there is at least one reliable option along the offensive line. Most teams with as high of expectations as the Colts are focused on the one weak spot, but for this team, it's more about the one strong spot. And that's Castonzo at left tackle, and more generally, the left side of the line with Castonzo and Thomas.
But don't think that Castonzo is just on this list purely because of need, because he's not. He's a pretty good offensive lineman too. He's best at run blocking, and no one doubted the team when they ran to the left more last year because they were running behind Castonzo, the team's best run blocker. His pass protection is also solid (although probably best suited for right tackle) and was a lone bright spot along a terrible line last year. Hopefully the latter isn't the case again this year, but either way, the Colts need Castonzo to continue to improve on the improvement he made last season and they need him to continue to be a bright spot along the offensive line.
And if you don't believe me that Anthony Castonzo is very important to this team's success this year, just imagine one thing: Andrew Luck down on the field injured. The Colts' season would then come crashing down in an instant. They absolutely must keep him as upright as possible, and it starts with Castonzo being as good as they expect him to be. Because with so many other concerns along the line, the last thing they need is yet another question mark.