The odd twist about the new guy stepping in to coach the Eagles defense is that Sean McDermott has actually been with the team longer than Jim Johnson. Longer, in fact, than Andy Reid or any of the players on the roster.

McDermott was hired way back in 1998, Ray Rhodes' final year as head coach. He was a kid then, fresh off a year as a graduate assistant at William and Mary, where he had played college ball. Reid kept him on at least partly because McDermott was still a blank slate, coaching-wise - one of the dedicated, hustling young guys Reid wanted to fill out his staff.

To fully get the idea behind all this, it helps to recall a staff photo from Mike Holmgren's first season as head coach of the Green Bay Packers. His 1992 assistants include five future NFL head coaches: Reid, Jon Gruden, Steve Mariucci, Dick Jauron, and Ray Rhodes.

Reid modeled his approach on Holmgren, who had modeled his on Bill Walsh. The main idea was to have very good coaches in the key jobs - offensive and defensive coordinator, special teams coach - and a group of developing young coaches in the pipeline, preparing for bigger things.

This is why we're all assured that, no matter how long McDermott stands in for Johnson, the Eagles will not miss a beat on defense. There is a reason McDermott was reportedly in the mix for coordinator jobs in New Orleans, Denver, and Green Bay.

But there seems to be a paradox here. If everyone agrees that Johnson has been vital to the team's success over the last decade - and they do - then everyone also has to agree that he can't be all that easy to replace, whether it's now or five years from now. Johnson is a coach's coach and, truth be told, he had more NFL street cred than Reid at the beginning of their partnership.

The Eagles point to precedent. No one outside the NFL really knew who Steve Spagnuolo was or what he did as an Eagles assistant. When New York Giants coach Tom Coughlin looked to change his defense's approach, he turned to a guy who had spent years learning from Johnson. Spagnuolo helped win a Super Bowl in his first year with the Giants. After his second, he was hired away as a head coach by the St. Louis Rams.

Then there is John Harbaugh, another of the young guys Reid retained from the Rhodes regime. After years as a very good special teams coach, Harbaugh asked to move over to the defensive side of the ball in order to improve his chances of becoming an NFL coordinator. He skipped that step after one year as Johnson's secondary coach. The Baltimore Ravens saw something in Harbaugh, gave him their head coaching job, and were rewarded with a trip to the AFC championship game in his first season.

So all's well, right? The Reid tree is a proven producer of high-quality coaches, so the Eagles have no worries.

Except, of course, that's not the way it works. Joe Montana played quarterback at Notre Dame. That doesn't make every Notre Dame quarterback Joe Montana.

McDermott may well be a terrific defensive coordinator and, eventually, a head coach in the NFL. There's no reason to believe he won't. But there's no way for anyone - not Reid, not Johnson, not McDermott himself - to know for sure.

McDermott passed a milestone yesterday, running the defensive side of a practice without the reassuring and commanding presence of Johnson. It was only a group of rookies and a couple younger veterans running around in shorts, but it was still a moment.

As per Reid's sometimes silly media policies, McDermott didn't talk with reporters after practice. So he was spared having to say the obvious, that he hopes Johnson's leave of absence is brief and his health improves, that he feels prepared to fill in for as long as necessary. And he didn't have to deflect questions that need asking but seem premature at this point, such as what elements he would bring to the defense.

If Johnson had retired, Reid could have surveyed the available candidates and gone with the best choice to keep the defense clicking at a championship level. Circumstances have created a more uncertain situation.

But you get the sense Reid would have promoted McDermott in any case, and that he would be next in line if Johnson comes back and coaches for a year or three or five - the scenario everyone would prefer.

Reid was right about Johnson. He was right about Harbaugh and Spagnuolo and Brad Childress. That doesn't guarantee that he's right about McDermott, but it certainly improves the odds.

Contact columnist Phil Sheridan at 215-854-2844 or psheridan@phillynews.com.