But the biggest mistake wasn't placing your future in the hands of a man that had never coached defense at the collegiate or professional level. That may have worked under normal circumstances.
It was handing the defense to a novice when it was fairly clear in February that much of the offseason and the opportunity for instruction were going to be canceled by the NFL lockout. It was asking Castillo to spend months working on a new scheme when he really had no idea who many of his players were going be.
Reid was obviously planning a major overhaul on defense as far back as February. And that's pretty much close to what he did when he brought Nnamdi Asomugha, Cullen Jenkins, Jason Babin and Dominique Rodgers-Cromartie aboard six weeks before the season was to start.
Reid was basically saying to Castillo, "Here's a bunch of talent. Make it work."
Thus far, it hasn't – not by long shot. And the Eagles' season is in jeopardy of slipping away following yet another second half collapse by their defense.
Sunday's was by far the worst. Up, 23-3, early in the third quarter, the Eagles surrendered 21 straight points to the San Francisco 49ers and lost, 24-23, at Lincoln Financial Field.
Two weeks ago, they coughed up a 10-point fourth quarter lead to the Falcons and lost. Last week, the Eagles were ahead by two in the fourth and lost. On Sunday they were up six and lost. Again.
In all, the Eagles has been outscored, 36-0, in the fourth quarter of its last three games, all losses.
"If it was just a defensive game, I would tell you yes," Reid said when asked if his defense bore responsibility. "But it's an offensive game, too. Nobody is pointing fingers at anybody."
They weren't in the Eagles locker room, at least the players interviewed weren't.
"This is [Castillo's] first year with a defense that he didn't get an offseason with," said Asomugha, a disappointment thus far. "Everybody is sticking by Juan and the calls he is making."
After the game, Castillo met with reporters. His eyes were sunken and swollen. Each week the questions about his readiness pile up.
"You'll see," Castillo said. "We'll get this turned around just like we have before, and [fans] can be happy that I did do the job."
Right now, he has only one way to go. Aside from the fourth-quarter disasters, Castillo's unit has been sieve-like against the run, horrendous in tackling and wretched in the red zone.
Opposing offenses have scored touchdowns on 8 of 10 trips inside the 20. The defense has allowed 10 passing touchdowns and is on pace to allow 40 this season.
Last season, Sean McDermott's group was historically bad allowing 33 touchdowns on 44 red zone possessions (76.7 percent). His 2010 defense gave up a franchise-worst 31 scores through the air. Those were numbers that led to McDermott's firing.
"We have the ability. We have the talent," Castillo said. "I got to do a better job putting the guys in better position to make plays."
Going into the game, Castillo's split-personality defense was torched in the first and fourth quarters but shut down offenses in the second and third. While it was once again woeful at the start and finish on Sunday, things got out of hand in the third quarter. The 49ers scored 14 points and gained 199 yards, numbers greater than what the defense allowed (10 points and 184 yards) in the third quarter of the previous three games combined.
And then there were the big plays. In the first three games, the Eagles surrendered nine plays of more than 20 yards. San Francisco – ranked 32d in the league on offense coming in - had six on Sunday.
It's still difficult to ascertain how much of the in-game blame game rests on Castillo's shoulders. He didn't miss on tackling Giants receiver Victor Cruz last week. He didn't get faked out of his jock strap by 49ers running back Frank Gore on Sunday.
But opposing offenses are making second-half adjustments, and the Eagles aren't countering.
There are 12 games remaining but only a few to fix the problems before Reid will likely be forced to make drastic changes.
What can be done? More personnel shake-ups? A schematic change?
"I don't know if you want to make changes schematically in a defense that is still fresh," Asomugha said. "It's not like this is a defense that this is our third year playing together."
Should someone else, perhaps defensive backs coach Johnny Lynn who has been a defensive coordinator before, be calling the plays?
It doesn't sound that nutty at this point.
Contact staff writer Jeff McLane at 215-854-4745, firstname.lastname@example.org or @Jeff_McLane on Twitter.