The reflex hammer struck the right knee.

Tap. Tap. Tap.

Then, the left knee.

Touch. Tap. Tap.

There was no response. Darryl Stingley was placed on a stretcher. The Patriots receiver never walked again.

Derek Stingley Jr. learned what football was at its worst by the time he was in kindergarten, watching the highlight of the vicious hit on his grandfather.

“Every time I go on the field, I think about him and my dad,” LSU’s star cornerback said before the Tigers’ playoff semifinal win over Oklahoma. “Injuries are part of the game. But thinking like that, playing scared, that’s not the way to play.”

On Monday, the freshman phenom learns what football is at its best, when the Baton Rouge, La., native attempts to bring LSU its first national championship in 12 years, facing Clemson at Mercedes-Benz Superdome.

The boy who idolized former LSU cornerback Patrick Peterson, and initially committed to the local power at 14, became a consensus First Team All-American in his first college season, leading the SEC with six interceptions and ranking second nationally in passes defended (21), while serving as top-ranked LSU’s primary punt returner.

“If there was an adjustment, it was from the time we broke the huddle to the time he was covering the receiver,” coach Ed Orgeron said of Stingley’s transition from high school. “He is one of the best cornerbacks I have seen.”

It was shortly after Darryl told his son, Derek Sr., that his speedy grandson (then 5 years old) was going to be “special” that New England’s former first-round pick died in 2007.

Darryl was 26 and coming off his best of five NFL seasons, when Raiders safety Jack Tatum delivered the devastating blow to the head of the defenseless receiver in a 1978 preseason game. Darryl broke two vertebrae, and spent the final three decades of his life as a quadriplegic, always considering the injury a “freak accident.” He encouraged his son to follow his sports dream.

Derek Sr. became a two-sport star, spending three years’ in the Phillies farm system and a decade as a defensive back — primarily in the Arena Football League — who enjoyed a three-day stint on the Jets’ practice squad during his lone brush with the NFL.

As a longtime Arena League coach, Derek Sr. brought his prepubescent son into team meetings and film sessions, allowing the gifted child to participate in drills. As a personal coach, Derek Sr. helped his son develop into an extraordinarily athletic cornerback, who played varsity in eighth grade and became the top-ranked recruit in the country.

At LSU, Derek Sr. regularly attends his son’s practices.

“I never had a player ready to play like Derek Stingley Jr., and I think a his father had a lot to do with it,” Orgeron said last week. “In fact, I know he did. I’m very good friends with his father. I have a lot of respect for his father. He’s an excellent coach, an excellent dad. He and Derek have a really unique relationship.”

Derek Jr. was part of the team before his college career officially began. After graduating high school early and enrolling at LSU, Stingley worked with the team prior to the Fiesta Bowl last season. In his first practice, the 17-year-old picked off Joe Burrow, the quarterback who soon would claim the Heisman Trophy.

“Derek is as humble a guy as I’ve seen. No. 1 recruit, sometimes they come in and they’re all cocky and loud, and Derek didn’t say a word for three months,” Burrow said. “At practice, I would go out of my way to throw Derek’s way to see if he was as good as everyone said he was. I can tell you, yes, he is. Yes, he is.”

He is better than he believed. It is better than he imagined.

“It’s crazy because, like, last year, I’m watching these people on TV, and now I’m lining up right across from them,” Derek Jr. said. “Like the coolest thing about it to me is whenever I go line up on the opposite side, seeing all the coaches that I see on TV all the time, and they’re right there by me.

“I didn’t think it would be like this. I figured I’d do alright, but not the way I’m doing right now. I never would have thought that things would end up this way.”