"We're still feeling our way around on how his knee is going to respond to catching three or four times a week and so forth," Mills said. "We're not 100 percent sure of where we're at yet. We're still talking to the trainers and talking to him after each game and seeing how he feels and how he's moving."
Mills said there has been some concern recently about the number of pitched balls that have been getting past Castro, which could be a product of him still getting back into game shape as well as getting familiar with the pitching staff.
Castro and catching coordinator Matt Sinatro worked on some drills last weekend to address it.
"It was about a week ago we were taking notice of it, and we've talked to him about it," Mills said. "I know what he's coming back from and know what he missed last year with the knee situation. We're getting a bit more familiar with how the knee is going to feel, how it moves, how the pitchers are pitching and so forth."