Cox played with the first unit as early as the second series of the game. He stood out with that unit again, just after the 2-minute warning of the first half, when the Eagles stopped the Ravens cold.
Cox ran a terrific stunt, switching from inside to outside with Jason Babin, which made Ravens quarterback Joe Flacco turn left, where Trent Cole was closing. Flacco had to throw the ball away.
On the next play, Cox pushed right guard Marshal Yanda into the backfield. Flacco had to settle for a pass to running back Ray Rice, for just 3 yards.
After just two games, Cox knew what to expect.
"Right before the half, he made two plays where he was really able to read their blocking scheme," said defensive coordinator Juan Castillo.
"I'm understanding it," Cox said.
Cox finished the afternoon with five tackles, one for a loss. The defense held the formidable Ravens attack to 23 points, 10 of them off turnovers. It should be said, only 10, since the Eagles turned it over four times.
The most remarkable numbers, though, were 51 and 73.
Cox played 51 defensive snaps. That is 73 percent.
He played more than any other defensive lineman. He played 14 more snaps than starting tackle Derek Landri, 19 more than starter Cullen Jenkins.
" 'Wash' has done a great job with him," Castillo said. "The kid knows he's special."
The Eagles considered him special enough to trade up three spots in the draft to secure him. They recently have endured modest disappointments on the defensive line, through injury or ineffectiveness. Having spent valuable picks on Brandon Graham, Daniel Te'o-Nesheim and Trevor Laws, Bryan Smith, Victor Abiamiri and Brodrick Bunkley, they should have cultivated a stable of home-grown players along the line.
They did not.
They have Cole, a cadre of mercenaries, no Patterson . . . and Cox.
"He has to play," Castillo said. "We count on him to make plays."
He made a few in Cleveland.
He made a few more Sunday.
Cox bulldozed left guard Ramon Harewood into the backfield and nearly intercepted Flacco's first pass of the second half.
He collapsed the pocket on the last two plays of the Ravens' next possession, after which the Eagles tied it.
Late in the third quarter, Cox sealed off the right side of the Ravens' screen blockers. Graham, the end on Cox's left, hit Flacco, while Cedric Thornton, the tackle to Cox's right, sniffed out the screen and pounded Rice, creating a third down that the Ravens did not convert.
Things hardly could have gone better. Certainly, the timing was perfect.
For the home opener, Cox hosted his mother, Malissa, and Bertha Moton, who is the mother of Melvin Baker, Cox's best friend. Melvin Baker died in a car crash in Mississippi in June. Cox calls Bertha Moton "Mamma." He considered Baker a brother.
Cox took his mother and his Mamma to dinner every night after they landed Wednesday. He took them shopping.
Then, on Sunday, he took them to work.
Bertha Moton cried when Cox took the field. She cried during the national anthem.
"I'm so proud. To see him out there . . . there's really no words for it," she said. "I had a few tear-dripping moments."
Since the accident, Cox routinely telephones Moton. He hardly ever called her before.
"She's my 'mom.' I'm her 'son.' Sunday, it was really emotional," Cox said.
On Sunday, Cox had work to do. He played with a directness that he did not show in training camp.
"It's night and day," veteran end Darryl Tapp said.
Certainly, a light has come on for Cox.
"Really, it's just understanding the blocking schemes, and understanding the scheme here," Castillo said.
Washburn and Castillo use a version of the Wide Nine alignment, which usually flares ends far outside the offensive tackles with the aim of disrupting play in the backfield. That leaves the defensive tackles with an inordinate amount of responsibility, especially when teams run; even moreso when teams show a pass play and then run.
The scheme is not that simple, Castillo insisted.
"Everybody thinks it's just rushing the quarterback. What it is is, you rush, then you crush, then you close," Castillo said.
That might be clever phrasing, but unclear. Castillo clarified:
"When you rush, when you get off the ball, if it's a pass, you continue rushing the quarterback. If it's a run, and an offensive lineman blocks you, you try to take that offensive lineman right back to the back. You crush. Then, if you rush, and the offensive lineman blocks down, you have to [avoid that block and] be able to close.
"He's really come a long ways in understanding our scheme and what we're trying to do."
Mainly, the Eagles are trying to get Cox on the field as much as possible. To that end, Cox is eager to refine his technique.
A year ago, at Mississippi State, he could bully blockers with his 6-4, 298-pound frame. Today, he concentrates on the precise footwork and handwork that marks the NFL's best defensive linemen.
"It could be where my foot is positioned, or where my hands are in my alignment, to taking that bad step," Cox said. "My thing is, if things start off bad, it's going to be a bad play."
Considering the support that surrounds him, and considering his violent, absorptive nature, Cox isn't likely to have many bad plays.
"Fletch has a lot of fight in him. The attitude. That nastiness you like defensive linemen to have," said Jenkins, who worries that Cox might be overcoached. "You don't want to get on a guy like that too much, take away his natural abilities."
It is a balance. Without Patterson, who might miss the entire season, Cox's performance will be crucial. He understands that - and how little else he understands.
"He's a humble dude," Tapp said. "He's willing to listen."
On Monday, Cox listened to Washburn, who offered qualified praise. Washburn told Cox only that Cox played better Sunday than he had in Game 1.
That is fine with Cox.
"I can't afford to take a step backward," Cox said. "I am humble. What got me here is not having the answer to everything."
For the moment, Cox might not have the answer. Soon, though, he might be the answer.
Contact Marcus Hayes at email@example.com.