Piastri, who is 32 points behind Norris in the championship, says he had been driving with a broken rib for the three races leading up to the summer break in Britain, Hungary and Belgium.
He revealed the injury on Instagram this week, but until Thursday had not said when it happened or how it had affected him.
He said it had been caused by his race seat not being a perfect fit, and that the problem began to emerge at the Spanish Grand Prix five races ago.
“You make the seat at the the start of the year and sometimes you get it a bit wrong and some tracks don’t expose it,” Piastri said.
“But going to Barcelona, Austria, Silverstone, they are three pretty hardcore tracks, so a bit of a pressure point, and eventually my rib broke up. It’s all good now.”
He said he had discovered the extent of the injury after the British Grand Prix.
“The scan was the day after Silverstone but it was definitely broken before Silverstone,” the Australian said.
“Definitely it was some point around Austria. I think it was probably a bit disturbed in Barcelona and then after Austria it was pretty painful, and then Silverstone was a pretty nasty few days.
“But we made some changes and it was already getting better in Hungary and Belgium, so it’s all back to normal now.”
