The 22-year-old German told reporters at the Bahrain Grand Prix, where he has been cleared to compete, that he had suffered three fractured thoracic vertebrae in a crash at the Race of Champions in Miami in January.
Wehrlein was immobilised for about four weeks afterwards, losing fitness at a key point in his pre-season preparations.
He never did more than 10 laps at a time in Barcelona testing in March, doing only half days at a time, and arrived in Australia with about a week and half of serious training under his belt.
Although he took part in practice in Australia ahead of the opening race, Wehrlein was replaced in Melbourne and China by Ferrari's Italian reserve Antonio Giovinazzi.
Some commentators and former drivers questioned his withdrawal, arguing that they would have raced through the pain barrier rather than risk losing their place on the grid.
"If it was just muscle pain, do you think Sauber and Mercedes would accept for me not to drive," Wehrlein said when asked about the comments.
Read the full article on eurosport.co.uk: Wehrlein broke three vertebrae in pre-season crash