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.
"I knew something was not right," he added. "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.