"It's actually been going pretty well," Castro said. "It's good to see that after I catch nine [innings] it gets pretty tired. I'm just trying to get back into it, but the next day it usually feels pretty good. That's a positive thing. I haven't had any real problems. Towards the end of games it does get tired, but the first seven innings or so it actually feels really good."
Castro, 24, missed all of last season following the injury, which occurred early in Spring Training when he stepped awkwardly on first base while running out a ground ball in Lakeland, Fla. He was set to be the Astros' starting catcher, a job he will take into the 2012 season.
The rehab process has been slow and tedious, but Castro finally has his legs under him and has no restrictions when it comes to swinging a bat or blocking balls behind the plate.
"I feel it when I get into a certain position," he said. "When I slide to block out on one, I might feel it. Overall, it's actually better than I expected. It's been responding really well."
With the start of Spring Training a little more than three months away, Castro is growing increasingly eager about the idea of getting to Kissimmee, Fla., and beginning the season. This will be Castro's first full season in the Major Leagues after making his debut on June 22, 2010.
"I'm excited to get all my strength back I've lost throughout this whole process and come back into Spring Training where I was last year and kind of pick up where I left off before I got hurt," he said.
Castro's offensive numbers in Arizona aren't great, but they don't matter much at this point.
He's there to get his knee back into shape, and it's hard to get much consistency with a baseball swing when you're getting eight at-bats per week. When he's not playing in games, Castro is catching in the bullpen daily.
"I really didn't get that many at-bats in the instructional league before I came out here," he said. "I'm still basically at the very beginning of Spring Training as far as my swing and all that stuff. I'm feeling more comfortable every day. It's good just to get at-bats and get that timing back before I get into the offseason."
When the AFL season is over, Castro will head back to his home in the San Francisco Bay Area and train at the SPARTA Performance Science Center in Menlo Park, Calif., where he trained last offseason. He'll be working on stamina and overall muscle strength.
"Once I get my legs back under me, it will make catching that much easier on my knee," he said.
Castro has had a chance to catch each of the Astros pitching prospects in Arizona -- left-hander Dallas Keuchel and right-handed relievers Jason Stoffel and Josh Zeid -- and has been impressed.
"I've caught a couple of starts from Dallas Keuchel, and he does a good job keeping hitters off balance and keeping guys uncomfortable at the plate and really pitching his game," Castro said. "Josh Zeid and Jason Stoffel have good stuff."
The knee injury has also prevented Castro from finishing his degree at Stanford. He took classes last fall and remains two classes shy of earning a degree in sociology with an emphasis on business and the economy.
"When I get the opportunity to finish, I'm definitely going to jump on that," he said.