The four-time Sprint Cup champion came out of retirement to replace Earnhardt at Indianapolis and Pocono the past two weeks and will again replace Earnhardt this weekend at Watkins Glen and then the following race Aug. 20 at Bristol Motor Speedway.
Earnhardt has missed three races -- Alex Bowman replaced him July 17 at New Hampshire -- and does not have a timetable for his return.
He said on his "The Dale Jr. Download" podcast Monday he has some days when his balance is better and then there are moments when it is not. He said he believes it has to do with a lack of gaze stability.
"The gaze issue and the problem with my eyes being able to fix on an object at a great distance and stay there with head movement -- that is the problem. When I move my head, I lose the object that I am trying to target," Earnhardt said. "That is hand-in-hand with the balance."
Earnhardt underwent further evaluation Tuesday at the University of Pittsburgh Medical Center Sports Medicine Concussion Program and took to Twitter to say doctors were encouraged with his progress later Tuesday.
Excited to report I had a great trip to the doctor today for evaluation. Doctors and I both are feeling positive about the progress.— Dale Earnhardt Jr. (@DaleJr) August 2, 2016
University of Pittsburgh doctors tell Earnhardt three weeks is a short time with his concussion symptoms, but they are treatable.
"I'm very impatient," Earnhardt said. "I want change now and I want improvement now so I'm consistently texting my doctor ... going, 'What can I do to get better tomorrow?'
"He's like, 'Look you have got to realize that this might be a process and you just have to keep doing these exercises.'"
The Sprint Cup Series has a week off after The Glen, with the regular season finishing up with races at Bristol, Michigan, Darlington and Richmond before the 10-race Chase for the Sprint Cup.
At 56 points behind the likely current Chase cutoff, the only way Earnhardt could be eligible for the championship is to come back and win a race before the Chase. His return even before then is questionable, as he said his visits to his doctors come every few weeks.
"Dale will be back when he's ready, and we're looking forward to that happening, but the priority continues to be his health and well-being. We'll keep our focus on that and let the doctors guide us," team owner Rick Hendrick said in a statement.
Gordon, who finished 13th at Indianapolis and 27th at Pocono, has four career Cup wins at Watkins Glen and nine overall on road courses. He has five career Cup wins at Bristol.
Turning 45 on Thursday, Gordon will make his 800th career start Sunday at The Glen.
"This whole experience is something that is one that I want to enjoy and take a little bit more lightly, but I can't," Gordon said Friday at Pocono. "It's just not in me. It's just not who I am.
"And so I wish Dale a speedy recovery. I'd like him to be back in the car. But at the same time if he's not, I feel like each time I'm in the car I'll be more confident and comfortable to get better results."