CONCORD, N.C. — Dale Earnhardt Jr. will miss the next two races with his second concussion in the past six weeks, ending the championship chances of NASCAR’s most popular driver.

The first concussion, suffered in a crash during an Aug. 29 tire test at Kansas, went undiagnosed until Wednesday, when Earnhardt was examined in Charlotte for lingering effects from Sunday’s crash at Talladega.

“I knew having those two concussions back to back was not a good thing,” he said Thursday at Charlotte Motor Speedway. “I knew to go see someone whether I wanted to get out of the car or not.”

Hendrick Motorsports said Earnhardt will sit out races Charlotte and Kansas, and Regan Smith will replace him in the No. 88 Chevrolet.

Earnhardt, who celebrated his 38th birthday on Wednesday, was injured in a 25-car, last-lap accident Sunday at Talladega. Because he was able to drive his car away from the accident — teammate Jimmie Johnson even caught a lift on the window back to the garage — Earnhardt was not required to go to the care center for an examination at the time.

Immediately after the race, he called restrictor-plate racing “bloodthirsty” and said he no longer had any desire to compete at Daytona and Talladega.

The wreck was at least the second hard hit Earnhardt has had this season. He struck the wall extremely hard during the Goodyear test at Kansas when his right front tire failed, an accident driver Brad Keselowski tweeted about moments afterward. 

Earnhardt, who attended a Washington Redskins exhibition game later that evening, was asked Thursday why didn’t he seek attention after the Kansas accident.

“Too stubborn” he said. “With the Chase coming up, if I was to volunteer myself for medical attention, I didn’t know how difficult it would be to get back in.”

He added: “I knew something was not right. But I decided to just push through. I’d had concussions before.”

Dr. Jerry Petty, a neurosurgeon who consults for NASCAR and also personally treats Earnhardt, said the driver was honest about his symptoms over the last six weeks. Earnhardt underwent an MRI on Wednesday and tests came back normal with no damage.

“He had no amnesia after either incident, which is very important,” Petty said. We’ll want to give him four, five days without a headache,” and then they’ll try to invoke a headache to see how he reacts before clearing him to race.”

Said Earnhardt: “I feel perfectly fine, but I don’t want to keep getting hit in the head.”

Earnhardt earlier this season snapped a 143-race winless streak dating to 2008, and many believed he was in the best position in years to finally win his first Sprint Cup Series championship. But he had a mediocre start to the Chase for the Sprint Cup and left Talladega ranked 11th in the field.

By sitting out the next two races, he’ll most certainly finish last in the 12-driver Chase race. 

Earnhardt will also end his streak of 461 consecutive starts, which is the fifth longest active streak in the Sprint Cup Series.

NASCAR strengthened its commitment to keeping drivers with concussions off the track in 2002, in part because Earnhardt admitted he was unable to fully concentrate or communicate with his crew chief after an accident at California. He self-diagnosed himself with a concussion, which he revealed weeks later.

NASCAR then said doctors at infield care centers could require drivers to undergo CT scans or MRIs if they suspected a concussion. Clearance to race after suffering a concussion is not given until after a driver obtains a medical release.

Nationwide Series driver Eric McClure missed six weeks this season with lingering effects of a concussion suffered at Talladega. McClure said the concussion he sustained in the May 5 race was the third of his career, one of the main reasons his doctors and NASCAR officials made him sit out for an extended period of time.

“There’s not really a set timetable for those things and that’s been the challenging thing,” McClure said after his June return. “That’s what kept me from coming back was the lingering symptoms. I really felt a couple of weeks ago, after the first two weeks of being away from the track, and having total brain rest, that I was ready. But (my doctor) felt like we needed to wait, and I respect that opinion.”