While there is no correct answer to the ultimate prospect question, "When is the right time for a call-up?", Chuck Finder of the Pittsburgh Post Gazette answers some readers' questions, explaining the Pirates thought process on answering this question.
The questions specifically deal with Pedro Alvarez and Jose Tabata, both of whom are expected to begin the season in Triple-A. Tabata has had 148 plate appearances at the Triple-A level, and Alvarez has a half-season at Double-A as his highest experience, so the club would have to be completely blown away by either's spring performances to consider keeping one or the other in Pittsburgh. This feeling is only further advanced by the fact that Alvarez is blocked at third base by Andy LaRoche (although that won't stop him once they feel he's ready), and Tabata has Andrew McCutchen, Lastings Milledge, and Garrett Jones in his way in the outfield.
Essentially, there is nothing either player can do to convince the club to keep them on the Opening Day roster, but a strong showing this spring could expedite their eventually call-up. With a good spring and a strong first half in Triple-A, both could be in Pittsburgh by the All-star break.

