Dale Earnhardt Jr has revealed his Talladega accident caused a second concussion, as he had already been injured in a testing incident at Kansas almost six weeks ago.
The Hendrick Motorsports driver will sit out at least the next two NASCAR Sprint Cup series rounds at Charlotte and Kansas after being diagnosed with concussion on Wednesday, following headaches in the aftermath of Sunday's last-lap crash at Talladega.
Earnhardt said he first felt concussion symptoms after a 40g impact in a crash at a tyre test on 29 August.
"You know your body and you know how your mind works and I knew something was just not quite right," said Earnhardt.
"But I decided to just try to push through and work through it. I've had concussions before and knew exactly what I was dealing with. I felt pretty good after a week or two, definitely 80, 90 per cent by the time the Chase started and by the time we got to Talladega I felt 100 per cent.
"In the accident at the end of that race I was hit in the left-rear quarter panel and it was sort of an odd kind of a collision where the car spun around really quick and just sort of disoriented me.
"It was not even half of the impact that I had at Kansas but it was enough to cause me some concern."
This week, Earnhardt decided to get himself checked by Dr Jerry Petty, a neurosurgeon and NASCAR consultant.
"I trust his opinion, that's why I went to see him," said Earnhardt. "He's been a friend of mine for a long time and he's helped me through a lot of injuries before, so I believe when he tells me I don't need to be in the car."
Earnhardt admitted he was "stubborn" in not seeking medical advice following his initial Kansas crash but feared missing races.
"I regret not seeing somebody after that happened. I was stubborn", he said. "I had concussions before and thought I knew what I was dealing with and felt that I was capable of doing my job.
"When you have a concussion, the symptoms can be really mild and then they'll typically go away after a couple of days and you feel perfectly normal.
"But then when you get in the car and you go around the track at a high rate of speed you start to understand that some things are not where they need to be and some reactions are not as sharp.
"You really can't get a measurement of that until you get in the car.
"With the Chase coming up, if I was to volunteer myself to get attention and be removed from the car, I didn't know how difficult it would be to get back in."
Dr Petty said that headaches are the main symptom Earnhardt has to recover from before he can consider returning.
Regan Smith will stand in for Earnhardt at Hendrick this weekend, with AJ Allmendinger taking the Phoenix seat Smith had been earmarked for after Kurt Busch's move to Smith's previous team Furniture Row.