The decision, combined with the choice to promote Michael Vick to franchise quarterback, will define the post-McNabb portion of Reid's tenure here.
Each bit Reid last year: Vick, by leading a turnover-prone offense; Castillo, by losing five fourth-quarter leads. Vick was coming back no matter what. But in Castillo, Reid has now made not just one audacious bet, he has doubled down, even when he had a chance to change course.
In the end, months of fevered speculation about Reid's choice ended in an anticlimactic text message that simply announced the hiring of new secondary coach Todd Bowles and a Reid news conference at noon on Tuesday.
Just about status quo.
Normally, that wouldn't make for a big headline. But the move and the reaction to it are less about the particulars and more about Reid, whose every decision splits Eagles fans into a hardened camps reminiscent of the red state-blue state divide.
Depending on your perspective, Reid is either showing the kind of patience that has marked his mostly successful run in Philadelphia and Castillo is bound to improve, or the head coach is displaying the stubbornness that has delivered three seasons without a playoff win.
More will be heard from Reid Tuesday, and no doubt he will assure listeners that Castillo was his first choice all along, despite more than one report that the Eagles were in the Steve Spagnuolo sweepstakes.
Reid will surely point to his defense's four-game improvement at the end of the season, when the Eagles surged to a final ranking of 10th in points allowed.
But those games had little meaning and little pressure, thanks in large part to a defense that ranked 22d in points after the first 12 games, the ones in which the Eagles had a realistic shot at the postseason. Their late rally reflected improvement, but against poor quarterbacks.
Putting too much emphasis on final numbers ignores what was seen in the critical moments that alter the course of a season.
Which isn't to say Castillo won't get better. There's an argument that continuity makes more sense than hiring a fourth defensive coordinator in five years.
Continuity, after all, was a big argument owner Jeffrey Lurie used for retaining Reid - though in theory the team is trying to avoid the continuity of declining win totals and early playoff exits.
But Lurie stuck with Reid, and Reid is sticking with Castillo, who once drove to Green Bay through a snowstorm to meet the man in line to become new Eagles coach, and plead for a job.
Castillo will have help. Bowles, by all accounts, did a good job in Miami. A longtime secondary coach, he has the kind of resumé that most defensive coordinators build before getting a big job, so perhaps Bowles can help steer Castillo through some rough patches.
Of course, that was also the theory when the Eagles had veteran Dick Jauron as their defensive backs coach and the young Sean McDermott as defensive coordinator, and when former defensive coordinator Johnnie Lynn was added to Castillo's staff last year. Neither one's experience made the difference, and both were quickly gone.
But this move ultimately isn't about the hiring of a secondary coach. This is about Reid and his future.
Whatever Bowles' role, the responsibility for the defense falls on the coordinator, and that's Castillo. Responsibility for Castillo, and his success or failure, falls on Reid.
In Castillo, he is trusting a loyal assistant who never hid from his challenge or critics. Castillo has incredible drive and admirable qualities.
But a nice story and improvement against Rex Grossman isn't going to save Castillo or Reid. For that to happen, Castillo's defense will have to turn into a playoff-caliber unit capable of doing enough to win against the likes of Aaron Rodgers and Drew Brees.
Otherwise, the Eagles will be looking at another year of status-quo results. If that happens, next year's coaching announcements should be far less routine.
Contact staff writer Jonathan Tamari at 215-854-5214, firstname.lastname@example.org or @JonathanTamari on Twitter.