Calero, one of the more dependable arms in the Cardinals' bullpen, was injured in the fifth inning of Saturday's 13-9 interleague win at Kansas City. He tried to field a grounder by Desi Relaford and turned sharply.
He was taken from the field on a stretcher.
"They're going to cut me Monday and I'm going to be out for the season," Calero said after the game. "I was trying to catch the ball and I stubbed my right foot and it popped."
"It's not likely to be a career-threatening injury, but there is no chance for him to participate for the rest of this season," said team physician Dr. George Paletta.
Calero, a righthander, was 1-1 with a 2.82 ERA in 26 appearances, including one start, allowing 29 hits in 38 1/3 innings, with 20 walks and 51 strikeouts.