"Just locating the pitches -- whatever [catcher] Bruce [Maxwell] calls, I try to put them in a good spot," Castro said. "I think that gave me the chance to get guys out."
Castro picked up two strikeouts, getting the leadoff batter looking in both the third and fourth. Castro also retired all six hitters he faced in just 25 pitches, an efficient effort that A's manager Bob Melvin noticed.
"He throws hard and he's got a good little split, and he believes in himself," Melvin said. "He did the two innings to where it was less than 30 pitches and remains available tomorrow."
Castro was called up after the A's traded relievers Sean Doolittle and Ryan Madson to the Nationals on July 16 and has been solid since, allowing two runs in nine innings of work across six games.
Castro has also shown that he can give the A's some length while pitching successfully, going two innings or more without allowing a run in his past three relief appearances.
Castro was key in Oakland's comeback win Sunday vs. the Twins, coming in with the bases loaded and two outs in the fourth and getting out of it. He then added two more clean innings as the A's bullpen threw a total of 8 1/3 scoreless innings as Oakland won, 6-5, in 12 innings.
"Obviously, the goal for me coming over here was to help the team. The way we can do that is to hold the game right there so the team has a chance to come back," Castro said. "Unfortunately, that didn't happen tonight, but that's the goal always, no matter the score."
Castro has struck out 11 in nine innings while allowing only one walk and five hits. If he can continue to pitch well, the 29-year-old could become a valuable piece for the A's as they try to build their bullpen for the rest of this season and beyond.
Alex Simon is a reporter for MLB.com based in the Bay Area. Follow him on Twitter at @alexsimon99. This story was not subject to the approval of Major League Baseball or its clubs.