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.
"I don't care too much what the others say because they didn't know my situation and were commenting on it ... if you don't know what injury someone had, you shouldn't criticise him.
"One broken bone is not something that is very easy to catch up in one and a half weeks and I had three broken vertebrae," he added.
Wehrlein, who raced for now-defunct Manor last year and has a Mercedes contract, said he had no worries about his debut for the team on Sunday, in a race which will subject drivers to extreme G-forces in sweltering conditions.
"Medical-wise everything is good. It's just the muscles around, they went away if you can't do sport for a few weeks. But they are rebuilding quite quickly," he said.
"I think I will be fine in the car, no pain. The track is also quite flat, not many bumps."
(Reporting by Alan Baldwin, editing by Susan Fenton)
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