Thornburg last started a game on Sept. 2, for Triple-A Nashville. He made his only Major League start on June 19 against the Blue Jays.
The Brewers just were not willing to risk further damage to Peralta's valuable arm.
"If there were nothing there at all, then there would be a really serious discussion about what to do," Roenicke said. "Still having a little bit -- and it's not bad, just a little bit -- of tightness, it's not worth doing."
Peralta exited an outstanding start in Cincinnati on Thurdsay complaining of tightness in his right biceps. He underwent an MRI scan on Friday and was cleared of serious injury, but his status for one last start was dependent on Saturday's throwing session.
The 23-year-old, No. 2 on MLB.com's list of the top 20 Brewers prospects
, was 2-1 with a 2.48 ERA in five Major League starts and one relief appearance this season, making all of the starts in September with a 2.25 ERA. It was an improvement from his numbers at Triple-A Nashville, where he was 7-11 with a 4.66 ERA in 28 starts.
Peralta has pitched himself squarely into contention for the team's 2013 pitching rotation, Roenicke said.
"He just had the one game where he was off and in that one game we didn't help him out, either," Roenicke said. "I thought he was outstanding. There's not too many guys that throw the ball the way he does, with that kind of velocity and movement with a nice slider. Impressive."
Peralta was all the more impressive because of how unimpressive
he was in the previous two Spring Trainings, particularly in 2011, when he was presented with an opportunity to begin the season as Milwaukee's No. 5 starter and had a nightmare outing against the Rockies.
Roenicke declined to rank Peralta against the team's other prospects, including right-hander Mark Rogers, a similar hard-thrower who has also impressed before the Brewers shut him down in a preemptive move to protect his arm. Rogers, 26, was 3-1 with a 3.92 ERA in seven Brewers starts.
"They both did a nice job for us," Roenicke said. "I don't want to start comparing them [but] their stuff overwhelms big league hitters. How often do you have guys that can do that?"
Mike Fiers, another young pitcher whose innings have been monitored, remains in line to start Sunday against the Astros, Roenicke said.
But the Brewers may decide to bump Yovani Gallardo from Wednesday's season finale if they are eliminated from postseason contention. Gallardo topped 200 innings and 200 strikeouts in Friday's loss to the Astros.