He spent the entire season learning -- from his on-field play, from then-defensive backs coach Marcus Robertson, from veteran cornerback Rashean Mathis in the locker room and then in the offseason with Rod Woodson, one of the best defensive backs in NFL history.
Slay heard from so many influences, and this was both good and bad. He picked up a ton of knowledge. He also had to decipher and implement it and that took a bit more time, resulting in the inconsistent rookie year he had.
Now back for his second season, he knows he needs to learn less. Undergraduate football school, for him, is done.
"I'm way more ready than I was last year," Slay said. "Just coming into the game, I felt like last year I approached it like my senior year in college. So like this year, my game feels a lot better, a lot more comfortable."
To that end, he already somewhat started football graduate school. During the offseason, he spent around a week with Woodson at the Hall of Famer's home in Pleasanton, California -- the second straight year he tutored Slay.
Woodson took Slay out to the field last season prior to the NFL combine. He needed to gauge the skill level of the incoming rookie to know where to start. This season, with Slay having NFL tape to work with and an understanding of the NFL, going to the field wasn't as imperative.
It didn't happen at all.
Woodson made his points verbally with an assist from his yard. They watched one or two games before leaving the television to go outside. There, Woodson would make almost all of the corrections he needed to in Slay’s game, then checked if Slay retained the knowledge. Then they’d go back inside and watch another game or two before doing it all again.
"The biggest thing for him was when he won at the line of scrimmage using press, he won the down," Woodson said. "Then when he played off, he was either sometimes too shallow and played in a no man's land. Instead of playing 7 yards or 8 yards off, he was like 4.
"To play some of these receivers in the National Football League 4 yards off, and you can’t play that way. He has to find a happy medium where he belongs when he's playing off-technique. I think he's pretty good at press and has confidence in press."
When Slay pressed, he could bump a receiver at the line and let his instincts overtake the plethora of information he was still attempting to process from his time at junior college, Mississippi State and in his first season with the Lions.
In doing so, Slay had to unlearn some things from college -- and perhaps high school -- that worked for him in the past. In the NFL, it wasn't going to help. He had to stop playing with his "butt to the sideline," as Woodson explained. It was a technique many college cornerbacks use to see the field.
Do that in the NFL and you're playing catch-up almost every play as receivers take advantage and run by you.
"He had several bad habits," Woodson said. "I don't know if he had those in high school, to be honest, but some of the things that were taught to him from his coaches weren't really good in college so he had to get rid of those. For him, what I have told him is coaches are going to tell you a lot of things he can do.
"You don't have to take all of them, you know. Even with me talking to him, I'm going to tell you a lot of things but don't try to take everything in or do everything."
Too often last season Slay did that. He had his old tendencies to dismiss, a cornerbacks coach trying to impart new knowledge and veterans trying to help through experience. Then the advice from Woodson.
It forced him to think more than react, to not trust what he knew before, because as he was taught different things, he wasn't really learning it or understanding it to an NFL level yet. Hence his almost-apprenticeship last season.
Already, the Lions see a more comfortable Slay.
"He's grasping things a little more," Mathis said. "It's still a learning process for him, but he's learning stuff more. He's communicating a little more."
He also is trusting himself more and potentially playing in a scheme more suited for him. One of the issues Woodson corrected in the offseason was Slay's positioning in off-the-line coverage. Simply, the rookie was never truly sure where to position himself on the field.
He might not have to do that as much this season as Slay and other defensive backs said the Lions will be more aggressive this season with their secondary play.
"I would ask him what did he see in the downs, what did he do wrong and what did he do right," Woodson said. "Then by the time we got done, I made him write down his positive traits, what he does well and then what he does bad.
"Then I told him what he does well, you don't need to work on those every day but need to diminish your weaknesses, and if you diminish weaknesses and play to your strengths, you're a pretty good player."
Right now, that might be Slay's most important lesson of all. He's already learned so much -- and knows he isn't done yet.
"How much do I need to learn? Yeah, you see a big difference into the game and how I practice," Slay said. "You just see it. I know so much more than I did last year."