"I have confidence in myself that I can play," Harbor said. "I feel I'm the best player I've ever been. I have confidence I can get the job done somewhere in this league.
"I feel like I've improved every year. I've gotten better. I've built more consistency. That was my big thing coming into the offseason. I'd watch myself on film and I'd have a really good block, then I'd have a subpar block. I'd have a really nice route, then run another one where I'd say, 'Why did I do that?' It comes down to focus and doing everything and focusing on details and executing plays."
Harbor got his most extensive playing time of the preseason Thursday night against the Jets, as Kelly rested most of his starters. He played a summer-high 36 snaps, spending the first half outside and the second half inside at tight end.
He didn't really do anything in the game on offense to help his cause, catching only one pass and dropping two others.
He was much more impressive on special teams, streaking downfield in the first quarter and bringing down Jets punt returner Kyle Wilson. Also was the first man down on a third-quarter punt, just overrunning the returner, Zach Rogers.
Now he can only sit and wait and hope his ability to play both inside and outside and on special teams will convince Kelly he's worth keeping around.
"It's kind of a coin-flip situation for me," the 2010 fourth-round pick said before the game. "I don't know what's going to happen, where I'm going to end up. I just tried to go out there and play hard and execute to the best of my ability."
It will all come down to numbers for Harbor and the rest of the Eagles on the roster bubble.
How many tight ends will Kelly end up keeping? Three? Or four? How many wide receivers? Five? Or six?
The first-quarter hamstring injury to tight end James Casey Thursday probably helped Harbor's chances of making the team, at least in the short term.
When Kelly told Harbor to start taking snaps at wide receiver earlier in August, it appeared to be a good thing for the 26-year-old.
While he was the fourth tight end on the depth chart behind Brent Celek, rookie Zach Ertz and Casey, Harbor also was the fastest.
Football is all about creating mismatches, and Kelly was intrigued by the possibility of putting a 6-3, 255-pounder with 4.5 speed on the outside. Having a guy who could play both tight end and wide receiver also would allow the Eagles coach to save a roster spot he could use at another position.
But Harbor had to prove he was a better fifth or sixth wideout option than Greg Salas or undrafted rookie Russell Shepard. Has he done that? Hard to say.
"That's one of the unique traits Clay brings," Kelly said the other day when asked about Harbor's wide receiver-tight end versatility. "The ability to play multiple positions certainly is beneficial to him. Maybe that separates him from some of the other tight ends, who maybe don't have the ability to go outside. I think that's a plus for him. His versatility is a plus for him."
Harbor played well in the Eagles' first preseason game against the Patriots, catching three passes for 47 yards.
He was moved to wide receiver before the Carolina game and had no receptions in 24 snaps. Last week, against Jacksonville, he played only five snaps and had no catches.
Wide receiver isn't completely foreign to Harbor. He played the position in high school and was recruited to Missouri State as a wideout before being switched to tight end two games into his redshirt freshman season.
But this is the NFL, not the FCS. And he was moved outside in the middle of the preseason, not at the start of spring OTAs.
"I feel comfortable [out there]," Harbor said. "Obviously not as comfortable as I do at tight end. But I feel I can do some good things out there. Use my size and definitely help in the blocking aspect of it.
"But I definitely feel more comfortable at tight end than I do at wide receiver. But there's similarities in route concepts and things of that nature."
If Kelly decides there is no room for Harbor on the roster, the Eagles likely will try to trade him to a tight end-needy team such as the Patriots or the Ravens before today's 6 p.m. cutdown deadline. They wouldn't get much for him, but something is better than nothing.
This much is certain: Harbor will have a job somewhere this season. If not with the Eagles, then with someone else.
"It says a lot about our [tight end] position group," Harbor said when asked about his roster uncertainty despite playing very well the last two seasons. "It's a really good tight-end crew.
"It's been good for me to work with them. I've been able to take pieces from each person's game, get to see how they do things. You're competing against each other, pushing yourself. Trying to be better than each guy. That only helps you."
On Twitter: @Pdomo