"I mentioned there's competition at positions," Reid said. "Left tackle is one of those positions."
The Eagles spent the past three seasons operating with the confidence that Peters was entrenched on the left side. After Peters suffered the initial injury, the Eagles invested in a five-year deal with Bell that leaves the team with an easy out after one season and re-signed Dunlap. Their hopes were that the two signings fortified the position.
The move on Saturday might just be a way of testing Bell's mettle before the season, with the idea that competition can provoke the best performance out of a player still learning his way around the offense and the organization.
"You don't want anybody [else] to be on the ones; you want to be the guy on the ones," Reid said. "On the other hand, it's fair to King to do that. He came back here under the assumption that he'd have the opportunity to compete. He's done well, and he'll get an opportunity. There's not a lot of separation between those guys right at this minute, so King deserves that opportunity."
For Dunlap, the opportunity is one that has eluded him since the Eagles selected the 6-foot-9, 330-pound tackle in the seventh round in 2008. Dunlap has mostly been a versatile backup throughout his Eagles tenure, only starting seven games.
The Eagles did not re-sign Dunlap until after Peters' injury, so he had not previously been in the team's plans to start - especially when the Eagles expected to enter 2012 with all five starting linemen from 2011 locked up to long-term deals.
"I guess they're trying to get a feel for who's going to be the best fit for the O-line," said Dunlap, who did not find out about the move until offensive line coach Howard Mudd told him on the field to line up with the starters.
Bell did not receive any explanation about Saturday's change. He called his adjustment to the Eagles offense "an ongoing process" and thought he played "all right" in six snaps, although that's a small sample size to form an evaluation. But he offered little reaction to the decision after Saturday's walk-through.
"It's not surprising," Bell said. "It is what it is. That's coach's decision. It's not my job to sit there and coach. It's my job to just play."
Still, Bell was signed to be the Eagles' left tackle and entered training camp with the starting job for a reason. Even when it was suggested to him earlier in camp that his five-year deal was viewed by the public as a one-year contract, he countered that he expects to be here for the length of the contract. So it shouldn't come as a surprise that when asked on Saturday if he still considers himself the starter, Bell affirmed the belief.
"If I'm on the team," Bell said, "I consider myself the starter."
When asked about the change on Saturday, quarterback Mike Vick acknowledged that Dunlap has spent time in Philadelphia and Bell is still learning the offense. Vick said that in time, Bell will become a "good player" for the Eagles.
The prevailing question is how much time. The Eagles did not shuffle the two players at any point in training camp until Saturday, the very first practice after the preseason game. Whatever prompted the decision - whether it was dissatisfaction with Bell's performance, the desire to motivate Bell or simply to see whether the offense is better with Dunlap - what's clear is that the Eagles have newfound questions at left tackle less than a month before from the season opener.
"King's been with us a while and deserves that opportunity and Demetress has done a good job, he's worked hard," Reid said. "I just want to make sure. I want to make sure everyone has their shot."
Contact Zach Berman at firstname.lastname@example.org or follow on Twitter @ZBerm.
Staff writer Jeff McLane contributed to this article.