PHOENIX -- The Athletics welcomed a power arm back to their bullpen Thursday morning, reinstating right-hander Santiago Casilla from the disabled list.

Casilla was dominant in three rehabilitation appearances, striking out seven and allowing one run in 3 2/3 innings, and was 2-0 with an 0.93 ERA in 21 games with the A's before right elbow soreness sent him to the DL May 16.

"Santiago threw the ball real well down in [Triple-A] Sacramento," manager Bob Geren said. "His velocity [came] right back. He threw 97 [mph] the last time down there."

To make room on the 25-man roster for Casilla, the A's designated veteran reliever Kiko Calero for assignment. Calero, who had only been active for two weeks after opening the season on the 60-day disabled list with a torn right rotator cuff, was 0-0 with a 3.86 ERA in five games. He had three hits, three walks and seven strikeouts in 4 2/3 innings.

Avoiding the ax Thursday was the junior member of the A's bullpen, sidearmer Brad Ziegler. The 28-year-old rookie has allowed six hits and one walk in nine scoreless innings this season.

"Ziegler has pitched very well," Geren said. "He's kept the ball down, he's a ground-ball guy. He's doing exactly what he did in the Minor Leagues."

The A's have 10 days to trade Calero, place him on waivers or release him. If Calero clears waivers and is outrighted to the Minors, he has the requisite service time to refuse the assignment and become a free agent should he so wish.

Calero pitched two scoreless innings Wednesday. Casilla last pitched Monday and was available to work in Thursday's game.