Castillo, an energetic coach who is held in high regard by Reid, said he embraced the challenge of moving to defense and calling plays for the first time in his career.
"All my life has been a risk," Castillo, 51, said, citing humble beginnings in Mexico and a coaching career that moved from high school to Texas A&M-Kingsville to the NFL. "My life has always been about proving people wrong. I think that's what Philadelphia is. Philadelphia is a bunch of people who work hard who are always put down or always want to be the best when people think they can't."
Castillo will have to disprove many skeptics. He played linebacker in college at Texas A&I and in the USFL, and began his coaching career on defense, but he has not coached on defense since coordinating for a Texas high school in 1989.
He is loved by his players and has coached Eagles linemen such as Jon Runyan, Tra Thomas, and Jason Peters to Pro Bowls. But the offensive line struggled in pass protection last season.
While the Eagles led the NFL in rushing yards per attempt, a tribute to Castillo's unit, the team also gave up 49 sacks, fourth most in the NFL.
Castillo, though, said he knows how to attack offenses from all of his time coaching on that side of the ball and studying opposing blitzes. For years, he said, he would talk about defensive schemes with the late Jim Johnson, advising the Eagles' former defensive coordinator about what types of pressure would work, and which would be picked up.
"That's when I started seeing, 'Oh, man, wait a minute,' if I got on that side, how dangerous I could be," Castillo said.
Much about the move was odd, primarily the fact that Super Bowl assistants with defensive pedigrees might soon be available for interviews and had expressed interest in the Eagles. Even Eagles defenders were in disbelief.
"I thought it was the right decision at the right time, so I made it," Reid said.
The move comes at a critical time for Reid, who has failed to win a playoff game in each of the last two seasons and needs to repair his defense to go along with a high-powered offense. That he chose to stay within the Eagles family showed a faith in both his organization and himself. Castillo's success or struggles will reflect directly on Reid's judgment.
Castillo takes over a defense that allowed 31 touchdown passes and was atrocious in the red zone. He does not plan to change the defensive philosophy or the style of players the Eagles seek.
"We're going to be fast and physical and we're going to be fundamentally sound," Castillo said. He said he would count on new defensive line coach Jim Washburn to lead an aggressive front four that can create pressure without the blitz.
"He's a sic 'em type guy, which is great for the players that we have. Really what I'm going to do is the back end is going to complement those four defensive linemen that are going to get up the field, create havoc, make plays."
Former defensive coordinator Sean McDermott espoused a similar philosophy, but the Eagles' front four struggled to get to the quarterback last year, contributing to the firing of McDermott and the departure of two defensive assistants. A third left for another job.
A key to promoting Castillo, Reid said, was the hiring of former Colts offensive line coach Howard Mudd, an accomplished coach with 36 years of experience. He was with the Colts from 1998 to 2009 and out of football last season.
He will be introduced Monday.
"You've got to put this whole thing together. There is more than one piece," Reid said.
The time it took to find Mudd might help explain why it took the Eagles nearly three weeks to finally hire a coach who has been with them the whole time.
"This wasn't a situation I had to rush into," Reid said. "I just wanted to make it right."
The Eagles still need to add linebackers and secondary coaches after their departures.
Castillo is well-regarded but not well known outside of coaching circles since Reid bars his assistants from speaking to the press. But the high-energy coach was unmistakable on the practice field, woofing and hooting at offensive and defensive players alike during blocking drills.
"You've got to get your players to be like you, to get your energy to rub off on them," he said.
His work ethic is renowned among Eagles coaches. When Reid was first mentioned as a possible new head coach, Castillo drove from Philadelphia to Green Bay, through a snowstorm, to lobby for a job on the staff.
One of his first assignments was serving as quality control coach under Jon Gruden, when he was the Eagles' offensive coordinator.
"I knew that when we got Juan he was special. He had the best work ethic of any coach I worked with," Gruden said. "While Juan might not fit into what others may think should be a defensive coordinator, he's a great coach, and great coaches coach no matter the situation."
Reid clearly agrees. He has bet heavily that he is right.
Staff writer Jeff McLane contributed to this article.
Contact Jonathan Tamari at 215-854-5214 or firstname.lastname@example.org. Follow him on Twitter at www.twitter.com/JonathanTamari.