There's much unknown about Maclin's situation. He was expected to be a major part of this offense. How the team will replace him remains to be determined. He is entering the final year of his contract, so his future with the Eagles is in question. And he had torn a ligament in that knee before, so whether he can maximize the potential he possessed when the Eagles invested a 2009 first-round pick in him also lacks a firm answer.
Maclin, 25, will undergo surgery Tuesday in Birmingham, Ala., by James Andrews, a renowned sports orthopedist. He will spend a few weeks in Birmingham before returning to Philadelphia to rehabilitate and be with his teammates.
General manager Howie Roseman said Sunday that the Eagles will focus on internal options to replace Maclin, and they will count on more than just their wide receivers.
"We're not only looking at just the wide receiver group; we're looking at the running backs, we're looking at the tight ends, and those are the guys we have high hope and expectations for," Roseman said.
Riley Cooper has stepped in for Maclin with the first-team offense. Cooper, 25, is entering his fourth season. He has 46 career catches for 679 yards and five touchdowns, primarily as the No. 4 wide receiver.
At 6-foot-3 and 222 pounds, Cooper offers size that could help the Eagles on the outside, although he lacks speed. Coaches believed he had a strong spring, and Roseman noted that Cooper has had the benefit of a full offseason and preseason after the 2011 lockout and a broken collarbone limited him the last two years.
"When you talk about Riley, we talk a lot about traits," Roseman said. "He's big, he's strong, he can go up and get the football. He can block. And for a big guy, he can drop his weight, he can run routes."
Jason Avant will continue in his role as slot receiver. Arrelious Benn has talent but a history of injuries, so his status is undetermined. Roseman said Damaris Johnson could have a role, and he mentioned undrafted rookies Russell Shepard and Ifeanyi Momah.
But the position to watch is tight end, where the team signed James Casey and drafted Zach Ertz. Both can be split wide and used in multiple tight end sets with Brent Celek. The offense was already expected to be tight end-centric.
"[Chip Kelly is] much more personnel-driven than even I thought observing him at Oregon," Roseman said. "It's going to be based on the guys who are producing at a high level. If that's the tight end position, they'll get more reps. If it's the receiver position, if it's the running back group, that's yet to be determined."
Roseman was sympathetic to this being Maclin's contract year. Maclin did not hold out or complain about his contract, in part because of the professional standard he tries to maintain, but also because he believed his value would increase with a productive season. He said "you can't think that way" when asked whether he had second thoughts about not seeking an extension, and said he will worry about the next contract when it comes. But he emphasized that he does not want to leave the Eagles.
In the short term, he is focused on his rehab. When Maclin tore his ACL in 2006 at Missouri, it happened at the same time of year. He was running 40-yard dashes by February, and participated in spring football practices without a brace.
Maclin also noted other players who have recovered from multiple torn ACLs, including his close friend Danario Alexander, who was a productive wide receiver with the San Diego Chargers even after four knee surgeries.
"I have all the faith in the world that I'll be even stronger than I was before," Maclin said.
The experience in 2006 is why he knew exactly what happened when he was on the turf Saturday. Yet on Sunday, he tried to stay positive when he stood on the sideline with his teammates. But the reality was also present: Both Maclin and the team will enter an autumn of unknown.
"I felt like this could be something special, something different, something we've never seen before, and I wanted to be a part of it," Maclin said. "It just [stinks] that I won't be able to go out there and play this year."
Contact Zach Berman at firstname.lastname@example.org. Follow on Twitter @ZBerm.