Manager Bud Black said Kelly's arm hasn't "bounced back" after his last couple of starts. The Padres don't want to risk anything with the 22-year old and will also hold him out of the Arizona Fall League.
Instead, Bass will make his first start since going on the disabled list in June with an inflamed right shoulder. He made seven relief appearances in September, though none lasted longer than two innings. Bass will be limited to 40-60 pitches, Black said.
"He's not stretched out like some starting pitchers, but he's got a starter's mentality," Black said.
Thornburg began the year in Double-A Huntsville before making his Major League debut June 19. He has also pitched out of the Brewers' bullpen and will be making his third start of the season.
Though the Brewers were eliminated from postseason contention Sunday, they remain upbeat about how they rebounded in the final two months of the season. At one point, they went on a 24-6 run that got them back into contention for the second National League Wild Card.
Manager Ron Roenicke said he and owner Mark Attanasio addressed the team Monday, congratulating them on their late run at the playoffs.
"It was just that I was happy with the way we came back and got back into this thing," Roenicke said. "A couple of months ago, we didn't think we would be to this point."
Brewers: Hart will miss Tuesday
First baseman Corey Hart has battled an injury to his planter fascia for the last few weeks. Though he played Monday against the Padres, he will miss Tuesday's game after tweaking the injury diving for a ball in the fourth inning.
Hart went 0-for-3 before being replaced by Travis Ishikawa at first base in the eighth inning.
Padres: Cabrera speeding toward history
Shortstop Everth Cabrera stole two bases in Monday's 5-3 loss, raising his season total to 43. He has been caught just three times, giving him a success rate of 93.5 percent. That is the best success rate in team history.
Cabrera leads the National League in stolen bases. He is trying to become the first Padre to lead the league in stolen bases.
Brewers pitchers have struck out 1,386 hitters this season. That is the second most in Major League history, trailing the 2003 Cubs, who struck out 1,404 batters.