Pettitte went on the list Monday with a muscle strain. He had been pulled from a recent game because of the injury.
Doctors said the strain is not related to an elbow problem that sidelined him for three weeks in April.
Pettitte said he started to feel tightness in his forearm during a throwing session Sunday. It was later decided to put him back on the disabled list.
"I was hoping it was going to be looser than it was," said Pettitte, who is 4-1 with a 3.38 ERA in seven starts this season. "This is uncharted territory for me. I've had a lot
of elbow injuries before, but not a muscle injury."
Pettitte signed a three-year, $31.5 million contract with the Astros in December after spending nine seasons with the New York Yankees.