First-choice Italy fullback Mattia De Sciglio hurt his right leg in a practice session Thursday and is being assessed for a muscle injury that could force him out of the World Cup.
The Italian team says De Sciglio was taken to a local hospital for tests. He was later ruled out of the opening Group D match against England at Manaus on Saturday.
Italy coach Cesare Prandelli has until 24 hours before the Azzurri's opening match to replace injured players.
If De Sciglio's injury is serious, Prandelli could draft in Andrea Ranocchia, who is with the squad as a 24th man.
The 21-year-old De Sciglio was expected to start at left back. If he can't play, Giorgio Chiellini could move over from center back and Leonardo Bonucci or Gabriel Paletta could play in the central defense.
Italy also faces Costa Rica and Uruguay in Group D.