"We were getting by. But until they take pictures you never know exactly what was going on," said Peavy, who had the MRI earlier Friday at Scripps Green Hospital in La Jolla.
"I thought I was good enough to be out there. Today showed a little more severity than we thought. I could have gotten ugly if we didn't [have the MRI]."
Peavy, who was 6-6 with a 3.97 ERA in 13 starts and was scheduled to pitch Saturday in an Interleague game against the Angels, will be placed on the disabled list on Saturday. Josh Geer will take his start Saturday. The Padres won't have to fill that spot in the rotation until June 20.
Peavy, who injured the ankle making an abrupt stop after rounding third base in a game against the Cubs on May 22, will wear the cast for a month, according to Padres general manager Kevin Towers.
Even if the cast is removed in a month, Peavy would have to demonstrate that the tendon is strong enough for him to resume pitching. If you add in a Minor League rehabilitation assignment, it could be the end of July before Peavy pitches again with the Padres.
"Even if he gets the boot [cast] off, he's not going to be able do a lot of weight-bearing stuff initially," said Padres head trainer Todd Hutcheson, who ordered the MRI when it became apparent that Peavy's ankle wasn't getting any better.
"We want to rest it as much as we can over the next couple of weeks. The key ingredient to getting this thing better is staying off of it."
Peavy is 4-1 since suffering the injury on May 22, with his lone loss coming June 2 when he allowed four runs in the first inning. That outing was cut short by an upper respiratory infection, not because of the ankle injury.
Peavy's ankle responded to treatment, but would always start bothering him again when he pitched.
"It was the side-to-side movements, bouncing around, fielding a bunt, rounding a base when he was a baserunner that got him," San Diego manager Bud Black said. "The straight on didn't affect him that much."
In his last start against Arizona on Monday, Peavy described the pain as "manageable," even though he wore a visible grimace on his face when running to first base after a sacrifice bunt in the game.
Hutcheson said that, if left untreated, the tendon damage could have been made worse.
"That muscle helps support the arch," he said. "When his foot is on the ground, and his weight is on it, that muscle has to be working in order for him to function properly. He was pretty heavily taped his last couple of starts.
"If you're not able to plant and stabilize on that foot, you're looking at his [pitching] mechanics being changed. It was time to get it evaluate and see what the results were."
Peavy was told he suffered no additional damage by pitching on the ankle during these past three weeks. He kept waiting for it to get better and spent extensive time with the Padres training staff. But there was never a time when he felt like he was getting better.
"It was no fun to run and cover first base," Peavy said. "But that being said, I thought I was good enough to be out there, compete and help the boys win. I thought I was good enough to be out there."