The third day of the 2015 NFL draft was about to unfold, and Stefon Diggs’s name remained uncalled. Michael Locksley, his offensive coordinator at Maryland, fidgeted. He knew Diggs’s pride and Diggs’s competitiveness. He knew with three rounds and 99 players already gone, Diggs would be seething.

“I was agonizing right there along with him,” Locksley said. “I remember thinking, ‘We got to do something to get his mind off this draft.’”

So that Saturday morning, the draft’s final day, Locksley grabbed three things: his son Kai, who was then a high school quarterback, some footballs and a boombox. He told Diggs to meet him at Maryland Stadium. They went down to the field. They flipped on some music. And for two hours, they caught passes.

“That’s how he cleared his head: working out, catching balls, running routes,” Locksley said. “He needed to work.”

There is not a straight line from that workout to Diggs’s central role in Sunday’s AFC championship game, because nothing in Diggs’s career has been a straight line. He is a Buffalo Bill when it seemed the Minnesota Vikings should keep him and cherish him for his entire career. He could have gone to college anywhere but stayed home to play at Maryland when such a decision was decidedly uncool. The Terrapins were so injury-riddled at quarterback when he was a freshman that he finished the year catching passes from a converted linebacker. He broke his leg as a sophomore, lacerated a kidney as a junior, turned pro anyway — and had to wait to celebrate.

By the time the Vikings’ turn came up in the fifth round, 145 players had been selected. Eighteen of them were wide receivers. The Locksleys and Diggs heard nothing through their impromptu workout, so Diggs headed home. The coach and his son headed to Buffalo Wild Wings for some lunch. Finally, there, Locksley’s phone pinged: The Vikings had selected Diggs.

“I can just remember thinking that anybody that knows the competitor Stef is, the Vikings have no idea what they’re getting,” Locksley, now Maryland’s head coach, said this week. “Not only is he a great player, but he’s got a little chip on his shoulder. There’s no way there’s 18 better receivers than this guy. But he had to go show it. He had to prove himself, which he had never really had to do before.”

The proof is on film every week, even deep into the playoffs. No one in the NFL caught more balls for more yards than Diggs did during the regular season: 127 passes for 1,535 yards. No one has caught more balls for more yards in the playoffs: 14 for 234 more yards in Buffalo’s two victories. Last year, Buffalo ranked 24th in the NFL in total yards and 26th in passing yards. This year, the Bills were second and third, respectively. The improvement of third-year quarterback Josh Allen is a significant factor. But Diggs’s arrival changed what the Bills could do on offense.

And it started with a tweet in March, when Diggs was still with Minnesota, with whom he had a turbulent 2019. The Vikings decided to extend quarterback Kirk Cousins for two more years — and $66 million more.

Diggs’s tweet that day: “it’s time for a new beginning.”

By the draft, he had it — in Buffalo, with Allen. The Vikings received four draft picks, including a 2020 first-rounder. The Bills got a seventh-rounder and the wide receiver who turned into a first-team all-pro. The deal not only changed the Bills. It changed how Diggs felt about himself and football.

“I probably have some moments where I lightly touched on it or lightly thought about it to myself,” Diggs said in a Zoom call with reporters this week. “It’s just so weird; it’s not even in the same atmosphere of me last year.

“I try not to get too much caught up in the past. I definitely will think about it when the season is over, and I hope it’s not over no time soon. I’ve kind of been putting it on the back burner or putting my thoughts on anything like that on the back burner just to kind of stay in the moment and relishing the moment because more importantly I don’t want this moment to pass me by.”

There is an edge to the way Diggs plays and an edge to the way he thinks. In the hour after he authored what might be the top play in Vikings’ history — the 61-yard catch-and-run from Case Keenum as time expired to beat New Orleans in the playoffs following the 2017 season — he said matter-of-factly: “Half the guys on this offense nobody really wanted. Late-round guys, guys that got a lot to prove with a chip on their shoulder.”

That fits perfectly with Buffalo as a town and the Bills as a franchise. Before the Bills beat Indianapolis and Baltimore in these playoffs, their last postseason victory came in 1995, when Jim Kelly was their quarterback and Andre Reed their featured wideout. Since then, Buffalo hasn’t had anything near that Hall of Fame pair. Until now.

“The only thing I can say is we’re very similar,” Allen said in a chummy joint interview with Diggs on NBC following the win over the Ravens. “We’re both competitive as hell.”

That fits with the player Locksley recruited from Our Lady of Good Counsel in Olney, Md., to College Park.

“It’s why he came to Maryland,” Locksley said. “He said: ‘I don’t need Ohio State to validate me. I’m a good player without Ohio State.’ He always creates that little edge within himself.”

Around the time of the trade to Buffalo, Locksley talked to Bills offensive coordinator Brian Daboll, with whom he once shared coordinator duties at Alabama. Daboll needed to know: What makes this guy tick?

“My opinion has always been that if he trusts you and if you show loyalty and honesty toward him, he’s going to give you everything he’s got,” Locksley said he told his old colleague. “He’s a loyal, loyal kid, a loyal, loyal person. As long as the trust is there, he’s going to work hard for you.”

Clearly, Diggs trusts Allen, whom he called “the general” and “the man.” He trusts Daboll, who dialed up plays that had Diggs targeted 166 times, a career high. And most of all, he trusts himself — just as he did when he bucked convention and went to Maryland, just as he did when he was taken behind 145 others, just as he did when he wanted a new start.

“Believe in yourself,” Diggs said. “That’s how I got here. That’s how the trade happened. That’s how all that happened: I believed in myself. I wanted more for myself, and I feel like God had something on the other side.”

Five of the wide receivers taken ahead of Diggs are out of the NFL. None have more than his 492 career receptions. And with the Bills just one win from the Super Bowl, none are alive this deep in the playoffs. Analyze the Bills-Chiefs matchup a thousand different ways. But doubt Stefon Diggs at your own peril.