KHARKIV, Ukraine -- Denmark midfielder Niki Zimling left the team's training session in an ambulance with an unknown injury Friday, just a day before its opening game at the European Championship.

Denmark was just finishing the session on the eve of its Group B match against the Netherlands when Zimling was carried out of the stadium in Kharkiv on a stretcher. His right boot had been taken off as he was lifted into the ambulance.

It wasn't immediately clear what kind of injury he had received.

Zimling, 27, is a regular starter in Denmark's lineup as a defensive midfielder alongside William Kvist.