"I told him, 'If you want to win, come to Philly,'" Cox told reporters on Tuesday. "I know what he can bring to our team. The character, the swagger, the hard work, the leadership."
Slay previously credited Cox, who was his teammate at Mississippi State in 2011, with selling him on Philly during an appearance on the Eagles Insider podcast in early April (h/t Dave Spadaro of the team's official website): "I'm very, very excited. Fletcher, man, he convinced me big time. He was like, 'This is the place to be. You're going to love it here.'"
Cox has enjoyed three straight playoff runs, including Philly's Super Bowl LII victory over the New England Patriots to cap the 2017 season.
Detroit has made the postseason twice since 2013—in 2014 and '16—but lost in the wild-card round both times.
On March 19, the Lions traded Slay to Philadelphia in exchange for third- and fifth-round picks in the 2020 NFL draft. The Eagles signed the 29-year-old to a three-year, $50 million extension upon his arrival.
Slay did not hide his desire to leave the Lions:
And the three-time Pro Bowler had an inkling that he was going to land with the Eagles.
"I knew Philly wanted me last year, tried to come get me last year, and I was hoping to come here," Slay told NBC Sports Philadelphia's John Clark on March 19. "I was pretty excited when I heard the news. Looking forward to coming down there and trying to perform the best I can and helping the team win."
He added: "I had a feeling they had their eyes on me for a minute. It was only a matter of time till it happened."
Slay enjoyed an All-Pro campaign in 2017, leading the NFL in interceptions (eight) and passes defended (26). Last season, he recorded two picks, 13 passes defended and 46 tackles (36 solo).
The Eagles lost cornerback Ronald Darby to Washington but added former Los Angeles Rams cornerback Nickell Robey-Coleman in free agency. Jalen Mills signed a one-year deal to remain in Philadelphia but will be moved from cornerback to safety.
Philadelphia ranked 19th in passing defense last season.