Busch, 44, who won earlier this year at Kansas, remains locked into the playoffs via a medical waiver.
He released the following statement on Wednesday, updating his recovery:
"Brain injury recovery doesn't always take a linear path. I've been feeling well in my recovery, but this week I pushed to get my heart rate and body in a race simulation type environment, and it's clear I'm not ready to be back in the race car.
"This was by far the hardest week emotionally because I do feel the progression of recovery, but racing requires an extreme physical and mental effort, and my body is not 100% able to sustain the intense race conditions. I am making progress and pushing hard each day. I am encouraged by my team of doctors, and we will continue to do everything it takes to get me to 100% to return to competition.
"Thanks to everyone for the continued support and best of luck to the 45 team and Ty (Gibbs) this weekend in Richmond."
Gibbs, who is also running fulltime in the NASCAR Xfinity Series, has been subbing for Busch in the No. 45 23XI Racing Toyota Camry. He impressed with a 16th place finish in his debut at Pocono, followed by a 17th at the Indianapolis Road Course, and even a top-ten at Michigan where he finished 10th.
Gibbs posted the following statement on social media: "My thoughts remain with Kurt and I wish him all the best as he continues to recover. He’s been a big help to me while I’m subbing and I hope to make him and the team proud this weekend in Richmond."
