Perhaps worried that it might become a habit, or perhaps just trying to let him not-so-subtly know that nobody is assured a roster spot, the Eagles expressed some interest in a couple of veteran free-agent tight ends last month, bringing in Visanthe Shiancoe for a workout and talking to Jeremy Shockey.
Whatever the purpose, Harbor, the Eagles' No. 2 tight end behind Brent Celek, got the message. He has had an outstanding camp, impressing the coaches with both his blocking and receiving.
"Clay is having a superb camp and he's doing some things that are really uncommon," offensive coordinator Marty Mornhinweg said. "He has really grown into that position and is doing a fine job there."
Harbor, a 2010 fourth-round pick out of Missouri State, is coming off a solid season in which he played 370 snaps, or 35.7 percent of the Eagles' offensive plays, which is a pretty decent workload for a No. 2 tight end.
While he only caught 13 passes, he contributed significantly to the success of a ground game that finished fifth in the NFL, averaging 142.3 rushing yards per game.
LeSean McCoy was selected first-team All-Pro after rushing for 1,309 yards last season. Of that total, 650 came out of two-tight-end sets with Celek and Harbor. McCoy averaged 5.7 yards per carry when the Eagles lined up in one-back sets with two tight ends and two wideouts.
As good as Harbor's blocking was last year, Morhinweg said it's gotten even better after having a full offseason to work on his technique with the coaching staff.
"He's a physical guy and his inline blocking has dramatically improved," he said. "[Tight-ends coach] Tom Melvin had a whole offseason with him, which was one of the keys for him, and it's really shown.
"He's always had that skill and ability coming out of college. He would hit you. He would hit the heck out of you. Now, he and Tom have refined his game, and he's really doing an outstanding job inline as well as with the pass game."
Said the 6-3, 250-pound Harbor: "I like to hit people and I like to try to be physical. In college, I played at a Division I-AA school. You could get away with that. You could just fire off and just try to physically outman somebody and move them and get the job done.
"Here, if you don't have the right technique, you're not even giving yourself a chance to win. Sometimes, you might have the right technique and the d-ends are just so good at what they do, they win. It took a lot of film, a lot of studying and a lot of good coaching to help me learn the proper technique."
The Eagles used a two-tight-end set 33.9 percent of the time last season, which wasn't nearly as much as the Patriots (77.6), but more than most NFL teams and more than any previous Andy Reid-coached team. And there is every indication that they will use it even more this season.
"Absolutely," Mornhinweg said. "We've got two outstanding tight ends. We're going to use all of our players' strengths, and that certainly would be one of them."
Harbor is hopeful that not only will his playing time increase, but also his involvement in the passing game. If the Eagles' training camp practices are any indication, Mornhinweg and head coach Andy Reid plan to attack opposing defenses from the inside-out this season, working the middle of the field with the tight ends, slot receiver Jason Avant and Jeremy Maclin, then going over the top to DeSean Jackson.
Celek was virtually ignored for the first five games last season, then caught 53 passes, including five for touchdowns, in the Eagles' last 11 games. Amazingly, he did that and led all NFL tight ends in yards after the catch despite playing much of the season with a sports hernia and a torn labrum in his left hip.
Harbor was targeted only 19 times last season, but is a former college wide receiver with 4.5 speed who set the school record for receptions at Missouri State.
Because both of them also are excellent blockers, it poses a dilemma for opposing defenses when they're on the field together. Do they stay in their base defense so they can defend against the run, or do they take out a linebacker and put in a fifth defensive back to defend against the pass?
"A lot of teams like to play [against] two-tight-end sets a certain way and we took advantage of that when they wouldn't play us a certain way," Harbor said. "We saw a lot of teams use nickel against other two-tight-end sets, because they weren't worried about getting beat in the blocking game.
"But with us, they had to go base defense, because they knew that both Brent and I could block and they'd get taken advantage of [if they played nickel]. So we got matched up base pretty much the whole season and that allows you to get mismatches in the passing game."
Despite that, they still were successful running the ball out of two-tight-end sets, as well as throwing it to Celek, who was the Eagles' top red-zone target, catching 10 passes and four TDs inside the 20. Add Harbor to the pass-catching mix and it will create an even bigger base-or-nickel dilemma for opposing defenses.
"I'd like to become a bigger part of the [pass] offense," Harbor said. "I know I'm capable of it and the coaches feel the same way, from what they've been telling me. I feel I'm a good mismatch for a linebacker in those situations. It took a while to get that confidence. But when you're playing with a team that's got the receivers we've got . . . "
The more playing time Harbor gets, though, the less Avant gets. Avant has established himself as one of the league's better slot receivers.
He had a career-high 52 catches last season and has had an outstanding training camp. Like Harbor, Avant has been catching everything thrown to him.
"He's one of the best slot receivers in the game," Harbor said. "So it's tough to say, 'I want to be in more,' because it's usually going to be him or me."
It's a win-win situation for the Eagles.
Contact Paul Domowitch at email@example.com. Follow him on Twitter @Pdomo. For more Eagles coverage and opinion, read the Daily News' Eagles blog at www.eagletarian.com.