COSTA MESA, Calif. -- Derek Watt might be the Watt brother you've never heard of, but he was the most celebrated as a high school player.

Watt, a fullback for Los Angeles Chargers, is the younger brother of J.J. Watt, who plays for the Houston Texans and the older brother of T.J. Watt, who plays for the Pittsburgh Steelers. Derek totaled 2,685 rushing yards and 44 touchdowns during his prep career. He also served as his team's kicker and punter, earning Wisconsin Player of the Year and USA Today All-America honors his final season at Pewaukee High School.

"They both say that I was one of, if not, the best high school football players they've ever seen," Watt said.

Of course, fast forward to the present and Derek's football fame has been eclipsed by the accomplishments of J.J. and T.J.

J.J. is one of the best players in the NFL, a perennial All-Pro defensive lineman and three-time defensive MVP. T.J., in his third NFL season with the Pittsburgh Steelers, is no slouch either, earning a Pro Bowl invitation as a pass rusher in 2018.

Both J.J. and T.J. were first-round selections; Derek was picked in the sixth round of the 2016 draft.

Derek says he and his brothers fantasized about making it to the NFL while playing football in their backyard. They pretended to play for their favorite team, the Green Bay Packers.

"We just continued to work and work, and finally we've all achieved the dream of getting to the NFL," Derek said. "And now it's just what our story is in the NFL, and how far it can go."

Derek will get an opportunity to play against older brother J.J. for the first time Sunday (4:25 p.m. ET, CBS). J.J. was nursing a back injury when the Chargers defeated the Texans in Houston three years ago.

The two never shared a football field on an organized team. J.J. was already on his way to college when Derek started high school football at Pewaukee. And by the time Derek arrived at Wisconsin, J.J. had declared early for the NFL draft.

"He Facetimed me [Monday] mainly to see my son," Derek said about his older brother, who enjoys spending time with 7-month-old Logan. "He wanted to see his nephew because he's starting to crawl now, so he wanted to see him. But he also was asking what the game plan was looking like this week and that type of stuff, and I'm not giving much away there. He's definitely trying to start the small talk there."

Derek is 2-0 against his brothers in the NFL. Along with beating the Texans three years ago, Derek faced off against T.J. last season in Pittsburgh, a 33-30 victory by the Chargers.

The middle brother, Derek is two years older than T.J. and four years younger than J.J. Derek and T.J. played together in high school and at Wisconsin. Derek said he played only a couple snaps on offense last season, so he didn't knock heads with his younger brother often.

The three remain close, communicating daily through group texts. Their parents, mother Connie and father John, will attend the game Sunday at Dignity Health Sports Park.

"It's very surreal," J.J. said. "Even just being at a meeting and seeing him on the scouting report and flipping on the film and -- it's just like any other week, you're watching film but then all of the sudden it's your brother on the film instead of just a fullback.

"It's very cool. I'm not going to try and act like it's not exciting or anything. It's awesome. I mean, we literally played football in the backyard, in the front yard, dreamed about playing the NFL. So, this is going to be very special."

While overshadowed by his talented, pass-rushing brothers, Derek has developed into a good player in his own right at the disappearing fullback position.

"As a fullback you're more of an adjuster than anything sometimes," Derek said. "The defense might do something you weren't expecting, and so you've got to make sure that you can still get to your guy and adjust off that. A guy might come free off his block and you've got to put him back on his guy, or just pick up somebody you weren't expecting to block."

Derek was never a fit as a pass-rusher, with T.J. saying Derek's destiny was to be in the scrum.

"They tried him out [at fullback at Wisconsin] in fall camp and it ended up working out in his favor," T.J. said. "He was more an inside linebacker. He’s not an edge guy. He was good in high school. He had a lot of tackles and had a lot of scholarships to play linebacker, so he was no slouch on defense. He's a plugger, just like he is a fullback. He'd be a run-stopping guy.”

Now Derek serves as the eyes and ears for current tailback Austin Ekeler, one of his good friends. Ekeler attended Watt's wedding to his wife, Gabriella, last year, and relies on Watt reading the defense on runs when he's in the game.

"It's a grind out here, especially on the practice field," Ekeler said. "So just having a friend out there with you makes you relax. We take a little pressure off each other because you have someone to talk to, put your mind at ease."

Derek is also tied for the team in special-teams tackles with Isaac Rochell.

"The best way to describe him is that he's just a football player," quarterback Philip Rivers said of Derek. "He can catch it well, he blocks, he's smart. He just kind of can do a little bit of everything."