DETROIT — Tracy Walker is a rare homegrown success story from the previous regime. He just might be the only homegrown success story on defense, a mid-round pick who developed into a quality safety and decided to return to the Lions despite drawing offers from multiple teams in free agency.

He signed a three-year, $25 million deal in March. He was named a captain for the first time last week.

Then three quarters into the season, he was ejected after drawing two personal fouls for unnecessary roughness. Detroit trailed 31-21 at the time, allowed another touchdown two plays later and lost 38-35 on Sunday against the Philadelphia Eagles.

“I just let my team down,” Walker said after the game. “Me, as a team captain, I’ve just got to be better. I was hot-headed at the moment, and like I said, I’ve just got to make better decisions at the end of the day.”

Sure does. Walker is in his fifth season with the club, making him the longest-serving player on the defense. He’s grown up so much over the years, and began shouldering a vocal leadership role in the final year of the previous regime. That’s a big reason why this regime — which has spent carefully in the early years of the rebuild — gave Walker more money than any other player in free agency.

He’s a good player with elite leadership skills.

“You belong here,” Campbell told Walker after the deal became official. “You were one of the guys we were talking about. When we get this thing right, all the (expletive) you had to go through, all the stuff you had to go through, it’s going to make it that much sweeter.

“So now this (defense) is yours. So what do you want to do with it? You’re one of these guys. You’re one of the core.”

Walker’s response: “Imma be one of those guys.”

Walker looked like one of those guys early against the Eagles. He had one sack on a safety blitz, defended one pass and finished with a team-high 11 tackles despite getting kicked out late in the third quarter.

Then again, Walker also lowered his head while making helmet-to-helmet contact with quarterback Jalen Hurts, who had already given himself up and was sliding. Then after getting pushed in the back while standing up, Walker turned around and struck tight end Dallas Goedert with his left hand, then also struck receiver Zach Pascal while a referee was trying to break up the fisticuffs.

Walker was flagged for unnecessary roughness for the hit on Hurts, flagged again for unnecessary roughness for everything that happened after it, and got the hook.

“I didn’t see what I was doing wrong,” Walker said. “Like I said, as I walked away, I got pushed in the back, so I reacted. For me, I’ve got to be better. Like I said, I let my team down. It’s a learning experience for me and I will be better from here on out.”

The Lions were already struggling to handle Philadelphia before the ejection, allowing four scores on five drives, three of which were touchdowns. Losing Walker — the club’s best defensive back — made coming back from a 10-point deficit that much harder.

Then Hurts connected with Goedert on a 22-yard pass on the very next play, Brandon Scott pounded the ball across the goal line on the one after that, and Detroit suddenly trailed 38-21 while replacing its best defensive back with JuJu Hughes, who had played 35 defensive snaps in his career.

“Unacceptable,” Campbell said of Walker, “and he knows that. That’s not OK. And that’s not what we’re looking for because that’s how you get beat, and those guys know that.”