Justin Verlander, the Astros' ace, has been diagnosed with a forearm strain and will miss at least two weeks, according to manager Dusty Baker.
The team's medical staff will evaluate him again after two weeks.
"We know it's a forearm strain," Baker said. "He's being seen by the doctors, so he'll probably be shut down for a couple of weeks, and we'll see from there."
Verlander just pitched Opening Day for the Astros, going six innings and giving up just two runs.
The injury happened early in the game, when Verlander mentioned to Baker that his arm was tender.
"You see he was throwing the ball great," Baker said. "It was kind of a shock to all of us."
The injury means Houston loses its best pitcher, while already trying to fortify its rotation.
Jose Urquidy, who was projected to be the No. 4 starter, missed all of summer camp and is training in Corpus Christi.
Verlander seemed healthy after undergoing groin surgery in March. He also suffered a lat injury during spring training.