No official word yet, but Zachary Levine tweets this afternoon that both Marwin Gonzalez and Carlos Corporan have joined the Astros today in San Francisco. This would appear to be bad news for both Jed Lowrie and Jason Castro.
Lowrie had his knee/ankle x-rayed, but no announcement has been made yet on the extent of the damage. Gonzalez had been in OKC on rehab assignment for a heel injury, so he'll presumably be activated when Lowrie is DL'd.
Castro, meanwhile, hasn't played since July 7, as he's been experiencing pain in his surgically repaired right knee. Corporan will be making his 2012 debut with the Astros after posting a .273/.339/.422 line in 51 games for OKC this season.
EDIT: Announcements now official - Castro to DL with right knee effusion ("water on the knee"), retroactive to July 8. Lowrie to DL with right ankle sprain and peroneal nerve injury. According to Johns Hopkins, that "affects a patient's ability to lift the foot at the ankle." But it's not an ACL, so hopefully that's good news...