The three-star prospect in the 2020 class tweeted that a recent evaluation by doctors discovered issues that could not be corrected by surgery.
Johnson is a pocket-passing quarterback out of Pinnacle High School in Phoenix and had appeared on the Netflix documentary "QB1: Beyond the Lights" with former teammate Spencer Rattler during their 2018 season.
The heart issue is called coarctation of the aorta, which was diagnosed when Johnson was 12. He was scheduled to receive an expansion adjustment to the original stent, but upon evaluation doctors found complications. Johnson then learned that playing a contact sport could rupture his aorta or limit the blood to his brain.
Despite the diagnosis, Wolverines coach Jim Harbaugh told Johnson his scholarship will be honored and he has the option of joining the Michigan staff in some capacity.
Johnson is still planning to attend Michigan and be a part of the football program.
"[Harbaugh] demonstrated a high level of loyalty to a player that will never throw a TD pass for him," Johnson said in the tweet. "I already bled Michigan BLUE, but Coach Harbaugh demonstrated an example to me, like the old 'Starfish Parable.'
"You cannot impact everyone's life, but sometimes making a difference in the life of just one counts for something."