Swiss captain Alex Frei injured his right ankle in training Wednesday only hours before the squad heads to South Africa for the World Cup.
Video footage on news website 20min.ch shows the FC Basel forward being treated on the pitch before limping off at Zurich's Letzigrund stadium.
National coach Ottmar Hitzfeld says Frei probably sprained the ankle but is being examined in hospital to check whether he has torn a ligament or broken any bones.
Hitzfeld told a news conference in Zurich he expects the 30-year-old Frei to accompany the team when it flies to South Africa later Wednesday
"Although the pain was very strong for him, I hope it's nothing serious," Hitzeld said. "Even if he can't make the first game, he can join us later in the tournament."
Frei is a key player in the Swiss team, which will take on Spain, Chile and Honduras in Group H.
Hitzfeld considers Switzerland the favourite against Honduras and expects his team to battle Chile for second place in the group.