Castro, who's been out two weeks, on Monday worked on agility drills to test the leg, and also caught a bullpen session.
"I want to make sure he can explode out of the box and leg out a double and score from second in the middle of a game without hesitation before we do anything," manager A.J. Hinch said. "He caught a bullpen today and did more blocking. We hope to have him available pretty soon."
Castro hopes to be able to play at some point during the Rangers series, which ends on Thursday.
"Maybe only -- hopefully -- a few more days would be my guess," Castro said. "That's kind of where I'm looking. Obviously, it's all dependent on how I feel, but the way things have been progressing, I don't think it should be too much longer."
Gomez was scratched on Sunday in Anaheim after experiencing discomfort in his left intercostal during batting practice. He was scheduled to undergo an MRI on Monday in Houston, and general manager Jeff Luhnow said the strain is mild compared with others his players have had.
Gomez is hitting .234 with nine doubles, four homers and 13 RBIs in 38 games since coming to the Astros from the Brewers in a July trade. He has hit safely in 14 of his last 16 games, batting .306 with seven doubles, three homers, nine RBIs and a .362 on-base percentage.
"Losing Gomez for any stretch is not ideal," Hinch said.
Gonzalez, who has gotten the lion's share of starts at first base recently, played in a lot of pain the last two days but was relieved that an X-ray showed no serious injury.
"Yesterday, when I got the infield hit, [it] broke my bat and my hand was shaking," Gonzalez said. "Hopefully I get better soon."
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.