The Yankees said that Pettitte will be treated conservatively and evaluated on a daily basis. Pettitte completed just five innings against Baltimore, hurling 77 pitches, before exiting with a lead and complaining of stiffness in his elbow area. He picked up his fourth victory in the Bombers' 7-5 win.
Pettitte was evaluated at New York-Presbyterian Hospital and, before the results were announced, offered a statement to Yankees director of media relations Jason Zillo.
"First and foremost, it's frustrating," Pettitte said. "You know how much pride I take in going deep into ballgames. Anytime you're talking about the area around the elbow, you start to worry a little bit. I think it's going to be fine, but I'll know a lot more after the MRI results come back."
The 37-year-old Pettitte added that he felt the stiffness was unlike his 2004 injury with the Astros, which required surgery.
"This is nothing like my previous injury with the flexor tendon," he said. "It's a completely different area of the arm. It's nothing like it at all."
Yankees manager Joe Girardi said that Pettitte felt some stiffness on the top of his left forearm in his April 30 start against the White Sox, and had received treatment then. He also had treatment before Wednesday's start, but the stiffness moved to the back of his arm near the elbow area.
"I think anytime a guy goes for tests, you're somewhat concerned," Girardi said. "The fact that [the stiffness] has moved spots makes me feel a little bit better. It was never in the ligament or the belly of the forearm. ... We'll wait to see what the test says."
Pettitte finally informed Girardi and the coaching staff of the stiffness after the fifth inning, although he did so reluctantly, wanting to stay in the game.
"We had to drag it out of him a little bit," Yankees pitching coach Dave Eiland said. "The expression on his face kind of told us the story. We don't want him to push it this early in the season."
Girardi said that Pettitte's command seemed fine, as he walked two and struck out two, an outing that hadn't necessarily raised a red flag.
"We've seen Andy pitch through a lot of aches and pains over his long career," Girardi said.
Eiland said that, as of Wednesday afternoon, Pettitte was to remain in the Yankees' rotation.
"Until we hear otherwise, he's our scheduled [starter]," Eiland said. "We might back his in-between side work off, but we have to wait and see."
With Pettitte's victory Wednesday, Yankees starters improved to 16-4 as a group in 2010. His rotation mates did not seem worried when informed of Pettitte's situation.
"Andy always finds a way," Phil Hughes said. "I'm not sure exactly what the deal is with him, but he'll find a way to get out there and get some wins."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.