Captain Tomas Rosicky, who has struggled with a calf problem, trained for a second straight day but Baros left the training pitch early after feeling tightness in his thigh, Czech team doctor Petr Krejci told local media.
"He stretched his abductor muscle," Krejci told local media as the Czechs prepare to face Russia in their opening Group A match in Wroclaw, Poland on Friday.
Baros, the Golden Boot winner at Euro 2004, has been earmarked by coach Michal Bilek to lead the attack at the tournament being co-hosted by Poland and Ukraine.
The pacy former Liverpool and Aston Villa striker has only scored three goals for the Czechs in the past two years but Bilek lacks alternatives and Baros, at his best, has the power to run through defences.
After playing Russia, the Czechs will face Poland and former European champions Greece in the group stage.
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