The 28-year-old backstop was not in the lineup for Tuesday night's 8-4 win vs. the Mariners a day after he was hit by a 94-mph fastball from Mariners closer Carson Smith in the 10th inning of the D-backs' 4-3 win over Seattle on Monday night.
Castillo, who homered in his first at-bat on Monday, was wearing an elbow guard after being hit in the same spot on Saturday, but he was unable to bend his left arm and was lifted for Jarrod Saltalamacchia, who caught the bottom of the 10th.
Castillo said before Tuesday's game that there was less pain and swelling in his left elbow and that he could be used as a late defensive replacement or pinch-hitter in Tuesday's game if needed.
"It's feeling better, it's still sore, but it's feeling better today. I couldn't do much with it [Monday], but whatever they need me to do in the game I will do," Castillo said. "I'll do my best. It doesn't feel really good, but I'm going to be available to play."
D-backs manager Chip Hale said he would ideally like to give Castillo the night off, but he said Castillo is lobbying hard to return to the field.
"If we need him late, he said he's ready to go," Hale said. "It's just unfortunate. Last night, he had a pad on it, but it was the exact same spot. He's pushing hard to get in there tonight and definitely play tomorrow. I'll evaluate him after the game and see how he feels."
Fellow catcher Oscar Hernandez started Tuesday's game. Castillo said he was told to hold off on testing his elbow by swinging on Tuesday but that he will continue to receive treatment to reduce pain and swelling.
"They told me to don't do anything today just until the swelling goes down and the soreness goes away," Castillo said. "I don't like to miss time. I like to play every day. I will just try to get better -- the swelling is down -- and I will just try to get ready for the game."
Andrew Erickson is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.