(CNN) -- Just after the U.S. team finished second in its 4x400-meter relay heat, a limping Manteo Mitchell told reporters his injury was nothing serious.
"It might be a simple cramp," he said.
Turns out it was a lot more than that.
Mitchell broke a bone in his left leg, USA Track and Field said in a news release. The 25-year-old, running the leadoff leg, felt his fibula snap with 200 meters to go. He had to finish or the United States would have been out of the event.
"I wanted to just lie down. It felt like somebody literally just snapped my leg in half," he said.
For a half-lap he ran in excruciating pain until he handed off to Josh Mance.
Mitchell's split was 46.1 and the Americans went on to finish in 2:58.87 to qualify for Friday's final.
Mitchell said on Monday he was going up a set of stairs, missed one and landed poorly.
"I got treatment and I was fine," he said, adding that he felt good warming up for Thursday's race.
An X-ray taken Thursday after the relay showed the break.
He seemed to have good spirits about it.
"Never thought that the phrase 'break a leg' would reach reality for me lol," he tweeted later.
USA Track and Field has not said who will take Mitchell's place on the team. Two other members of the relay team pool -- Jeremy Wariner and LaShawn Merritt -- have injured hamstrings and cannot run.
The Americans will likely turn to one of their 400-meter hurdlers. Angelo Taylor, who was fifth in the 400 hurdles, ran in the 4x400-meter relay final at the world championships last year. Kerron Clement ran for the U.S. at the Beijing Games. But that was in a qualifying race while Taylor ran in the final, winning gold.