"I've never had anything like this before," Castro said. "It doesn't feel too bad, so I'm hoping for the best right now. I'll get some treatment on it, and we'll see how it is in the next few days. We'll know more then."
Castro was holding out hope for good news.
"I'm cautiously optimistic right now," Castro said. "It doesn't feel too bad, so we'll see."
Castro suffered the injury while trying to beat out a ground ball in the seventh inning and landed awkwardly on first base as he converged with Detroit first baseman Miguel Cabrera. Castro began limping and was helped off the field by Triple-A Oklahoma City athletic trainer Mike Freer.
"I was just running down the line and kind of tried to avoid the tag, and I was kind of turning," Castro said. "When I did it, my foot stepped on the side of the base and stuck, and my upper body kept turning. It just felt like a little twinge, a little bit of pressure in there. I just wanted to walk on it. That's where I'm at right now."
Houston general manager Ed Wade was in Tampa, Fla., watching the Astros lose to the Yankees, 6-5, but the GM was made aware of the injury via a phone call by head athletic trainer Nate Lucero.
"We know he twisted the knee, and at the very least it's a sprain," Wade said. "We'll get it treated appropriately. It's too early to push any panic buttons. We'll wait and see what the MRI shows tomorrow and address it accordingly."
Astros manager Brad Mills said he had planned to give Castro the next two days off from catching and had scheduled him to catch one of Saturday's split-squad games.
"That's very much in jeopardy now, simply because we're going to have to wait and see," Mills said.
Castro, 23, reached the Major Leagues last year as part of the Astros' youth infusion following the team's slow start. In 67 games, Castro hit .205 with two homers and eight RBIs, but the catcher impressed Mills with the way he played defense and handled the pitching staff.
The Astros are hoping Castro is able to make strides offensively this year and settle in as the starting catcher. The Stanford University product said he's never had a knee injury in the past.
"Obviously, it's frustrating," Castro said. "It's one of those things that just kind of happens. It's really not anything you can prepare for. I'll try to do everything I can to get back as soon as I can."
Castro, who prides himself on staying in peak shape, can only await a diagnosis following Thursday's scheduled test.
"I felt good coming in and felt good today," Castro said. "We'll just see in the next few days how it progresses and take it from there. Hopefully, I can get back as soon as I can and get my reps in, and be ready as soon as possible."
The Astros entered camp with six catchers, three of whom are on the 40-man roster: Castro, Humberto Quintero and former top prospect J.R. Towles. Castro had been penciled in as the starter, with Quintero serving in the backup role he held all of last year.
In the offseason, the Astros signed Carlos Corporan to catch at Triple-A and serve as an insurance policy if something went wrong at the Major League level. Houston also has several young catching prospects on the rise, including Ben Heath, Chris Wallace and Federico Hernandez.