Red Sox chief baseball officer Chaim Bloom announced the injury on Wednesday, saying it will be "weeks, not days" before Sale can resume throwing.
Sale, who turns 33 on March 30, suffered the injury before the conclusion of the lockout during a pitching session at his alma mater, Florida Gulf Coast University. He said he initially felt discomfort, but over the coming days, the pain increased and led to an MRI, which revealed the stress fracture on his eighth rib. When the lockout ended, Sale notified Bloom and manager Alex Cora of the injury.
There is no timeline for Sale's return.
Sale, like many players, controlled his own offseason workouts and dictated his own preparation -- and in the aftermath of his injury, he was left to seek a diagnosis and treatment on his own. But Sale refused to blame those circumstances as a contributing factor for his injury.
"What would have happened if I was training here the whole offseason? That's an unanswerable question," he said. "So why even waste time on that? I was given a set of circumstances Dec. 1, and I had to play with what I had to play with. And that's exactly what I did... It was the same for everybody. It wasn't just me. Everybody had to deal with this."
Since Sale signed a five-year, $145 million extension with the Red Sox in March 2019, the lefty has missed significant time to injury. The seven-time All-Star missed nearly two years due to an elbow injury that eventually required Tommy John surgery. He has not made more than 30 starts in a season since 2017, his first year with Boston.
"I'm just kind of waiting. I'm like a dog on a chain right now," Sale said. "I can't wait to get off this thing. You know, the last couple years have sucked and I've run into some pretty unlucky circumstances with arm troubles and then my neck and then this. But what can you do?"
The rib injury leads to small things being difficult, according to Sale. Sneezes, coughs and any movement requiring rotation cause pain. And while he has pitched in just nine games since the 2019 season, Sale said he was not concerned about his ability to remain a consistently healthy pitcher.
"The arm stuff that bothered me a lot and I looked at that as a failure for me," Sale said. "Knowing what I know now, injuries happen. There's not much you can do, and especially with this, this was a freak incident. ... I've never even dealt with anything like this. The way my body was feeling before all of this happened, I mean, I was throwing off a mound a lot better than I had been at that time in previous years."
While Sale won't be available when the Red Sox open their regular season April 7 in New York against the rival Yankees, the lefty views this latest injury as another hurdle to overcome.
"I've got work to do," Sale said. "I've been behind much bigger hurdles, I've been behind rocks. I thought I'd never be able to get over hurdles. This is just another one of them. Another life test I guess to get over and all the while trying to be a good teammate and a good leader."
Sale also informed reporters that he has not received a COVID-19 vaccination -- and at the moment, that would disqualify him from playing in games in Toronto, because of a mandate for visitors to Canada. But it's unclear when he'll be able to pitch at all, after an offseason in which all of the AL East contenders have invested in improvements.
ESPN's Buster Olney contributed to this report.