Houston forward Ching suffered the injury in an MLS game against Real Salt Lake last week and faces around a month out of action after a scan showed tearing.
"The MRI (scan) revealed a little bit more than a mild tear. They said the tear was on the surface of the muscle, but it was a long tear. I'm thinking three to four weeks, but it may be longer," the player told Major League Soccer's website.
The U.S. faces the Czech Republic on May 25 and then Turkey four days later in pre-World Cup friendlies.
While not a guaranteed starter, Ching has featured in qualifying and with Charlie Davies having been sidelined following a car crash, the U.S are short of options in attack.
Head coach Bob Bradley has to submit his preliminary squad of 30 players by May 11 - when Hawaiian Ching could still be out of action.
"Hopefully I can get back on the field as quick as I can and get back up to fitness and that's all I can do at this point," said Ching.
"My goal is to get back by the end of April, early May, because I feel that would give me the best chance to get back on the World Cup team and, of course, help the (Houston) Dynamo."
"The hamstring injury has set me back and has hurt my chances a little bit for the World Cup, which is frustrating.
"I spoke with one of the assistant coaches [of the U.S. team] who inquired about how bad it was ...I think being back at the end of April, beginning of May is pretty realistic," he said.
Ching was part of Bruce Arena's squad at the World Cup in Germany in 2006 but did not feature.
The U.S face England, Slovenia and Algeria in group C at the finals in South Africa.
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