New England Patriots coach Bill Belichick, a friend of Kelly's, has done similar things in the past. Receivers Troy Brown and Julian Edelman have played defensive back in games. Linebacker Mike Vrabel caught 10 career passes as a red-zone tight end. The Eagles' experimentation is not unprecedented, although it is uncommon at the NFL level.
Harbor went through one practice at linebacker. It was considered a "Phase 2" day, so it was not offense vs. defense. There was no offensive installation that day - Harbor called it a "review day" - so he was not falling behind with the offense. He remains in the tight-end room during meetings, but he was in the linebacker meeting room for one day.
Harbor was a wide receiver and a safety in high school, and has no experience at linebacker. At 6-foot-3 and 252 pounds with a 4.69-second 40-yard dash in the 2010 NFL scouting combine, Harbor offers a combination of size and speed that seemed to intrigue the coaches enough to try him as a pass rusher.
It also could be a way to preserve a spot on the roster. Harbor was the No. 2 tight end last season, but the Eagles added James Casey in free agency and Zach Ertz in the draft to complement Brent Celek. Even though Kelly plans to feature tight ends, there's a crowd at the position.
Harbor has 47 catches for 421 yards and four touchdowns in his three seasons since the Eagles took him in the fourth round in 2010. His roster spot appears vulnerable, even though the Eagles are not expected to carry a fullback.
"Obviously, we've got a lot of tight ends here," Harbor said. "I know there's a good chance that I might not be here, but I'm just going to play as hard as I can, learn what they need me to learn, do what I need to do to make this team, make this team better."
Avant did not see any different roles in practice as evidence that his spot might be in jeopardy. A steady contributor and valuable locker-room leader during his seven NFL seasons, Avant excelled in the slot under Andy Reid. But he is 30 years old and slated to make $2 million this season, and Kelly might have a different idea from Reid of what he wants in a receiver.
"It's one of those things that I work hard, and whatever that may be, and whatever the team wants me to do, I'll do it," Avant said. "Whatever that is - special teams, offense - whatever it may bring. I'm going to do it the best I can. I don't necessarily think it's a clouded future. I think it's an honor, in any way, for any coach to think you can do other things."
Avant has been versatile throughout his career. He has played inside and outside receiver, although he has been considerably more effective on the inside. He continues to contribute on special teams, has been an emergency quarterback, and has even taken practice reps as a holder on field goals.
"I can do a lot of different things," Avant said. "Always have. Already in this offense, I know all the positions already. So I think that's one of the things, as a receiver, I can play out or in. Always have been able to."
It's a new world at the NovaCare Complex under Kelly. Versatility is an attribute he especially values. The position experiments are the latest examples, and another week of OTAs could continue to deliver the unexpected.
Contact Zach Berman at firstname.lastname@example.org. Follow on Twitter @ZBerm.