"Hey," the fan, hanging over the railing, yelled, "I've got something for you."
Intrigued, Waller approached.
The nameless, faceless fan pulled something from his pocket and handed it to Waller, thanking him for putting his message out there on such a public platform as HBO's "Hard Knocks" series this summer. The object was a homemade "chip," a metal sobriety coin complete with the Raiders' logo, the words "To Thine Own Self Be True" and "Sober Nation," and the Roman numeral "II" to commemorate Waller's two years of being sober. A prayer of support was emblazoned on the back.
"It was just really cool that somebody went out of their way to have that made," said Waller, who keeps the coin in his backpack. "That was incredible. People that are fighting the same fight as us. It's a serious thing. It's a brotherhood. It's a sisterhood. We try to look out for one another, and that was just a small token of it. You learn how to be generous in recovery, so that was incredible."
The Raiders' tight end's story of addiction and recovery went viral when "Hard Knocks" cameras followed him to a meeting. He discussed the drugs he was hooked on -- "opiates, Oxy, pills, stuff like that, Xanax, cocaine" -- and how as a sixth-round draft pick of the Baltimore Ravens in 2015, he was limited to a combined 16 games since the start of the 2016 season due to failed drug tests.
Waller said he began taking pills as age 15.
"Sophomore year in high school," he said. "It was in a friend's medicine cabinet, and you like the feeling of it, you keep doing it, and for me, I wasn't as anxious, I wasn't worried about what everyone thought of me. It was a numbing tool. I don't want to feel all these things I'm feeling. That's how I started."
How bad did it get?
"I feel like if I were to continue down the same path, I probably wouldn't be alive, or I would be in jail or in a mental institution, probably," he said. "It was that bad."
Waller, who served a four-game suspension to start the 2016 season, was slapped with a year-long suspension by the NFL for the 2017 season.
"My life is completely out of control," Waller reflected. "I think I have willpower, but I really don't. I really need to use the tools necessary to get help. It was more so to save my life than to check off boxes for the league."
The day after his 25th birthday, on Sept. 14, 2017, Waller went to a four-day detox center in Boston. Then he went to Borden Cottage rehab center in Maine for a 30-day stay.
"That was a life-changing thing," he said.
Waller said the centers were presented and funded by the NFL.
When he walked out of Borden Cottage, football was not on his mind. He was grateful to the league for helping him, but he was not sure if he wanted to play again, especially in the high-stakes and fast-paced world of the NFL, if he wanted to maintain his peace.
It took a good six months for Waller to feel like he wanted to give the NFL another shot and stay clean.
Enter the Raiders. After being reinstated and spending most of the 2018 season on the Ravens' practice squad, Waller was signed to Oakland's 53-man roster on Nov. 26, 2018.
The converted receiver won the starting job in training camp.
"It's one of the great stories in football, one of the great things I've seen in my career. Really proud of him," Raiders coach Jon Gruden said of Waller. "So thrilled for him getting it together and being an honest, upfront guy and talking about it. Giving other young people the same enthusiasm to beat it. The same confidence that they can beat whatever addiction that you might have."
Raiders quarterback Derek Carr said he had "tears" in his eyes when he saw Waller's story on HBO.
"You know how many people that helped by being that vulnerable?" Carr said. "To see someone that vulnerable and that open ... imagine how many more people could get out, how many people we could help. To know that story and to watch this guy play on 'Monday Night Football' and make these crazy plays ... I mean, you have to root for that guy.
"I told him, 'Bro, you don’t know how many people that story, your story, is going to bless and help and change their life. Maybe today they'll go and ask for help. Maybe tomorrow they finally realize, man, if he could do it, then I can do it.' ... It's a powerful thing, and I think that needs to be talked about more."
Waller entered the season with 18 catches for 178 yards and two touchdowns in 22 career games. Through the first five games this season, he has 37 receptions, which leads all NFL tight ends, for 359 yards.
He had 13 catches for 134 yards at Minnesota in Week 3.
"He's really a rookie playing tight end. He was a wide receiver in college," Gruden said. "He sat out football for a year. He leads the league in receiving right now at that position, and if you watch him block, I don't know if there's a better tight end in football, really."
Waller shakes his head at his lot. Things could be worse -- much worse.
And temptation is always around the corner. Waller tries to attend at least two substance-abuse recovery meetings per week.
"It's difficult, but it's easier now because I know, I feel like I have a purpose in my life," he said. "I feel like I live with an intention. Before, I didn't really know who I wanted to be or what I stood for or what I was trying to accomplish.
"I'm trying to change the course of generations in my family that have been having the same habits."
Why share his journey on so public a forum?
"Just to give the human aspect of it," Waller said. "People see us as superhumans, but we're far from that. I have regular, everyday struggles, like someone else would. There's life on the other side of what you’re going through, and I feel like it would be a shame if I didn't share that because then people would go on suffering in silence. Just like I did. It's just ... paying it forward.
"I've come a long way, but at the end of the day, it's all about [stacking] days, and you go to meetings, and there's people in there with 20-something years. It's like, you're doing good things, but it's a lifelong commitment. Don't rest on it. Don't get complacent with it."
On the football field or in life.