The 28-year-old rider, who won the final BMX racing event at Rio 2016, was one of the favorites to clinch gold in Tokyo.
However, his third Games were cut short after he appeared to get tangled with another participant when competing in the third round of the BMX Racing men’s semifinal event at the Ariake Urban Sports Park.
He was transferred to the ICU of St Luke’s International Hospital, but is now rehabilitating in Salt Lake City.
“I improve a little each day which is all I can ask for,” he said in the post. “I am so thankful for my team of people for their endless support.”
Fields confirmed he will not be attending the BMX World Championships, but says he is “looking forward to watching the races.”
The competition, which will take place in the Netherlands, is scheduled to start on August 21 and runs through to August 22.