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.
Hendrick Motorsports announced Thursday that Earnhardt will sit out races in Charlotte and Kansas, with Regan Smith taking over the No. 88 Chevrolet.
Earnhardt, 38, was injured in a 25-car, last-lap accident Sunday at Talladega. Because he was able to drive his car away from the accident, Earnhardt was not required to go to the care centre for an examination at the time.
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 the second hard hit Earnhardt has had this season. He struck the wall during the Goodyear test at Kansas when his right front tire failed.
Earnhardt won for the first time this season ending a 143-race winless streak dating to 2008. He has not performed well in the Chase for the Sprint Cup however, currently sitting in 11th place.
Missing the next two races means Earnhardt will likely 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.