The corresponding roster move will have San Diego placing right-handed reliever Kevin Cameron on the 15-day disabled list with a first-degree sprained ulnar collateral ligament of his right elbow.
Cameron suffered the injury three pitches into his appearance in Tuesday's 5-3 loss to the Braves, though Cameron indicated he experienced some discomfort in the elbow prior to Wednesday.
The 35-year-old Estes was 4-2 with a 3.67 ERA in six starts for Triple-A Portland. He'll pitch out of the bullpen, Padres manager Bud Black said.
Estes, who has won 99 Major League games in his career, was an All-Star in 1997 as a member of the San Francisco Giants.
He has an out-clause in his contact that allows him to become a free agent if he's not on the Padres' 25-man roster on May 15.
Black said Tuesday that left-hander Wil Ledezma will start on Thursday, though he also said that it doesn't mean Ledezma is now the Padres fifth starter. The Padres would not need their fifth starter until May 13 in Chicago.
As for Cameron, it's not known how long he will be on the disabled list. Catcher Michael Barrett went on the disabled list with a similar elbow sprain on April 8 and only recently started playing catch.
Cameron had a 3.60 ERA in 10 games. He was recalled from Portland on April 15. The former Rule 5 pick started the season with Portland because he was sidelined in Spring Training with a hamstring injury, a broken left thumb and the flu that caused him to lose 12 pounds.
"He's a little sore ... I haven't spoken to him at length," Black said of Cameron.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.