Cameron suffered the first-degree sprain while throwing only three pitches in relief in Tuesday's game against Atlanta.
Estes was 4-2 with a 3.67 ERA in six starts for Portland.
"I think he's best suited to start, but we'll see how it plays out," said manager Bud Black before Thursday's game.
Black said Estes would be in the bullpen for Thursday's game against the Braves.
Estes made his last major league appearance for San Diego in 2006, when he was 0-1 with one start. Estes, who missed the 2007 season while recovering from Tommy John surgery, was 7-8 with a 4.80 ERA in 21 starts for Arizona in 2005.
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