Castro was placed on the disabled list retroactive to July 8, so he will be eligible to return on July 23, and he said it's reasonable to think he will be back that quickly because the injury isn't serious.
"I've been told this is somewhat normal for coming off the surgery I've had," said Castro, who had surgery to repair a torn ACL in March 2011.
He said he had felt some swelling over the past week and it just wouldn't subside. Castro said he could hit, but couldn't catch.
Castro is hitting .254. He had started 49 of the Astros 88 games at catcher.
The Astros purchased the contract of catcher Carlos Corporan, and he was in the lineup for Sunday's game against the Giants. Corporan was hitting .273 at Triple-A Oklahoma City, including 12 hits in his last 25 at-bats.
The Astros moved right-hander Kyle Weiland (shoulder) to the 60-day DL to make room on the 40-man roster for Corporan.