"I can't really figure out what's going on," he said. "I've been getting some treatment on it, and it's a little better. It's kind of a day-to-day thing right now. We'll see what's going on and what the actual diagnosis might be."
Castro, who throws with his right arm, admitted the pain was pretty bad both late Sunday and on Monday morning.
"It's kind of strange," he said.
The injury comes at a bad time for Castro, who was hitting .462 (6-for-13) in his last three games after getting moved into the No. 2 spot in the batting order. With Castro out of action, Carlos Corporan started at catcher Monday and super utility rookie Kike Hernandez was ready to step in as backup catcher.
"This guy becomes more valuable by the minute," Porter said.
Brian McTaggart is reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.