On Friday, Castro showed the electric right arm that has led the organization to believe that he might not be long for the Minor Leagues.
In a Double-A game in Surprise, Ariz., Castro struck out all six batters he faced in two shutout innings.
Castro appeared in five Cactus League games for the Padres before he was reassigned to Minor League camp. He posted a 1.93 ERA and three strikeouts in 4 2/3 innings. He also allowed one home run.
Castro is coming off a season in which he was named the Padres' Minor League Pitcher of the Year. He went 10-6 with a 3.33 ERA in 28 games for Class A Fort Wayne, helping the TinCaps to the Midwest League title.
He struck out 157 batters in 140 1/3 innings and allowed only 118 hits with 37 walks.
"He's got a great body, loose arm, a fastball in the mid-90s and a strikeout-type slider," said Padres director of player development Randy Smith. "His changeup is his third pitch. He's worked hard on that. Last year his command was leaps and bounds from where it was.
"He's been a special kid since Day One. His makeup, work ethic -- it was evident when we first signed him what kind of kid he was."
The Padres haven't officially decided where Castro will open the season, but he could skip over Class A Lake Elsinore and open with Double-A San Antonio.
Smith had high praise for Castro, even comparing him to another high-ceiling Padres pitching prospect.
"He's got high expectations of himself," Smith said. "He's got a very bright future. I said a year ago that he could be the [Mat] Latos of 2010, but only time will tell."
"He's going to have some bumps in the road, but he has got a chance to be real special. He's very business-like. He's not easily satisfied -- all the things you're looking for."
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.