Calero is a graduated from University Gardens High School in Puerto Rico and attended Miami-Dade College Kendall Campus.

Mainly a starting pitcher in the Royals minor league organization, he spent 6 years at the Single-A and Double-A levels before finally reaching Triple-A Omaha in 2002.

He made his Major League debut on April 2, 2003, for the Cards against the Milwaukee Brewers, he worked two thirds of an inning, allowing two hits and one walk but no runs.

After the 2004 season, Calero was traded from the Cardinals along with starting pitcher Dan Haren and top-hitting prospect Daric Barton to the Oakland Athletics for Mark Mulder.

The Rangers signed Calero to a minor league contract and assigned him to Triple-A Oklahoma, where he spent the rest of the season.

He has a street named after him, on a little part of Río Piedras called Fair View, where Calero grew up and played Little League ball.