The reason? The Eagles didn't get enough production out of their slot receiver, and with a group of receivers and tight ends that can line up almost anywhere, Kelly's offense should become even more difficult to defend.
Jason Avant, bless his leadership, professionalism, and hands of glue, is no longer a fit in Philadelphia. He did a lot of dirty work blocking last season, but he caught only 50 percent of his targeted passes (31 of 62 for 404 yards) out of the slot, according to Pro Football Focus.
The year before, he caught 76 percent (51 of 67 for 598 yards). Maclin, conceivably, would be the primary slot receiver, although in Kelly's system each ball catcher would be capable of lining up inside.
That is why Kelly has his receivers know all the routes on every play. He wants versatility, and that is why Cooper and likely Maclin are back and why Avant's days here are nearing the end.
"It doesn't necessarily have to be that one guy's playing inside," Eagles general manager Howie Roseman said Thursday after locking up Cooper with a five-year contract. "When we look in the draft and we grade players, it's inside-outside versatility. The more guys that you have that you can move around, that are movable parts, the harder it is to defend your offense."
Cooper isn't as much of an inside threat as Maclin, but he caught 9 of 12 targeted passes for 177 yards and two touchdowns inside last season. Maclin had similar slot numbers in his first three seasons but ran 50 percent more routes out of the slot in 2012 and caught 27 of 39 targeted passes for 359 yards and two touchdowns.
Jackson showed last season that he could be just as dangerous inside as out. He caught a career-high 28 of 34 targeted passes for 373 yards and two touchdowns inside in 2013.
But Ertz was almost as productive percentage-wise, catching 15 of 20 targeted passes inside for 178 yards and two touchdowns. With his athleticism and size (6-foot-5, 250 pounds) there is potential for Ertz to replace some of Avant's slot routes.
Maclin, though, would take over his fair share. He and the Eagles have to agree on a contract first. He wants a one-year, "show me" contract so that he can prove he's back from the second torn anterior cruciate ligament of his career and get a sizable long-term deal.
The Eagles would prefer a two- or three-year contract at considerably less to avoid having him reach the open market a year from now. They have 11 days to work something out before Maclin hits free agency.
If Maclin walks, the Eagles could opt for another free-agent receiver, but the field is barren and overpriced. They could draft a receiver in the early rounds, but Roseman insists the Eagles are sticking to their best-available philosophy, and that may not net them one.
Still, even if Maclin returns, the Eagles could take one of a handful of talented receivers in the first round.
The Eagles should have salary-cap flexibility within the framework of Cooper, Jackson, and Maclin's contracts to sign a first-round receiver and, if warranted, feature him in 2015.
Cooper essentially has only the first two years of his contract guaranteed at around $4 million per. The Eagles and Maclin could agree on a mutual option for Year 2. And Jackson's $9.75 million contract for 2015 is not guaranteed.
We're getting ahead of ourselves here, but the Eagles have enough maneuverability to morph their wide receiver corps if need be. And the same goes for their scheme.
Cooper said he probably would be the No. 3 receiver if Maclin returned, but there isn't really a pecking order in Kelly's offense. The Eagles used three or more receivers on 68 percent of their plays last season.
"We definitely have the chips in place," Cooper said. "Hopefully, we can get Maclin and get this thing going."
Cooper and Maclin aren't top-10 receivers. But they're more than capable, and with Jackson, the structure of Kelly's offense, and the restrictions of the salary cap, they are cost-efficient pieces for the Eagles.