After throwing a first-pitch strike to Durham's Reid Brignac with one out in the fourth inning, Hampton called toward the dugout, prompting Triple-A Richmond manager Dave Brundage and trainer Mike Grauss to visit the mound.
Moments later, the 35-year-old southpaw exited the game and put his return to the Majors in even further doubt.
"He was throwing good and then he just started feeling it during those last three pitches," Braves general manager Frank Wren said. "He just thought it was best to come out."
Braves manager Bobby Cox chose to take an optimistic approach and say that he hoped Hampton had pulled himself from the game just in time.
"I'll wait and see when we get home," said Cox, whose team was going back to Atlanta after Wednesday afternoon's series finale against the Nationals. "I was hoping that he'd make one more [rehab] start and then we could go from there."
Entering the fourth inning, Hampton had allowed two hits and thrown 57 pitches. With his target pitch count at 75, he was likely entering his final inning of the afternoon. In fact, before he took the mound in the fourth, Chuck James had started warming up in the bullpen.
Hampton, who hasn't pitched a Major League game since Aug. 19, 2005, was hoping to make one more additional rehab start and then join the Atlanta rotation on May 10. His regular-season debut had been scheduled for April 3. But approximately 10 minutes before throwing his first pitch that day, he strained his left pectoral muscle.
Wren said it is too early too determine the consequences of this setback. But it seems logical to believe the southpaw won't be able to rejoin the Braves' injury-depleted rotation within the next couple of weeks. His only other rehab start had come on Friday, when he tossed three scoreless innings for Richmond and felt minimal discomfort in his pectoral region.
Before traveling to Durham on Tuesday afternoon, Hampton indicated that he was still feeling some discomfort with the muscle that rests just under his left shoulder. In addition, numerous times over the course of the past couple weeks, he has said that he is still battling the fear of suffering another injury while pitching.
Hampton, who is in the final sesaon of his eight-year, $121 million contract, has missed the past two seasons recovering from separate surgical procedures on his left elbow. He made just 12 appearances in 2005 before undergoing Tommy John elbow ligament replacement surgery.
Hampton finished Wednesday's outing having allowed one run on three hits and two walks over 3 1/3 innings, with four strikeouts.
Mark Bowman is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.