A bad injury sustained by striker Jan Koller took the gloss off Czech Republic's 3-0 victory over United States in their opening Group E game on Monday.
"Unfortunately his injury is very serious," coach Karel Brueckner told reporters. "It is the only negative thing from today's match.
"It's probably a thigh muscle," said Brueckner, adding it is too early to tell how it would affect Koller's involvement in the rest of the tournament.
The 33-year-old was stretchered off in the 43rd minute after being hurt in a race for the ball with a US defender.
"He was in good shape and this is really bad luck," said Brueckner.
Koller, his country's all-time record scorer, has only recently returned after suffering a bad knee injury in September.
BULLET HEADER
He opened the scoring for the Czechs on Monday with a bullet header from close range in the fifth minute.
Man of the match Tomas Rosicky, whose two goals and fine overall performance were the driving force behind the victory, said: "We may have lost Koller and that makes this game tough to take.
"But we're really satisfied with the win and how we played. We had problems early on, we couldn't get our timing right.
"It improved halfway through the first half and I think we controlled the ball much better after that."
Rosicky said the match was not as easy as the result made it appear.
"They were tough opponents," he said. "I am not sure the score indicates how tough they were."
US coach Bruce Arena had a different view, criticising his players especially playmaker Landon Donovan.
"Landon showed no aggression," said Arena.
