Olympic 400 metres champion LaShawn Merritt suffered a slight hamstring strain when he pulled up in the Monaco Diamond League meeting with a cramp and is undergoing treatment, his coach told Reuters on Saturday.
"Everyone is hoping it was slight," Loren Seagrave said in an email.
American Merritt stopped running about 250 metres into his final pre-Olympics test at Monaco on July 20 when he felt a twinge in his left hamstring.
"I felt it kind of shake," the 26-year-old said at the time.
"After you have a cramp you have that soreness, but the fact that I can do this (flex his leg) is reassuring."
Merritt, the year's fastest 400 metres runner, and Grenadian world champion Kirani James are joint favourites for the London gold.
Merritt lost by three-hundredths of a second to the Caribbean runner at the 2011 world championships in South Korea.
His health is also concern to the American camp for the 4x400 metres relay in which he is a vital member of the squad.
Seagrave said Merritt was scheduled to arrive at the Games later on Saturday.
"He will continue to be treated and rehabilitated by the USATF (USA Track and Field) medical team in London," the coach said.
Seagrave is due to meet up with Merritt on Tuesday.
"It is planned he will do a training session that afternoon," the long-time speed coach said.
The first round of the 400 metres is scheduled for August 4.