Argentina international Lucas Biglia could miss the World Cup after fracturing two vertebrae during AC Milan's 1-0 defeat to Benevento on Saturday.
Biglia had to leave the pitch 20 minutes from time after being hit in the back by Nicolas Viola's knee.
Tests on Sunday revealed that the injury could end the midfielder's season and casts doubt over his World Cup involvement.
"AC Milan inform that Lucas Biglia reported a severe lumbar trauma due to an injury during Saturday's match against Benevento," a statement on the club's website said.
"The player has undergone medical examinations that highlighted a fracture of the transverse processes of the first two lumbar vertebrae.
"A detailed prognosis will be issued in the coming days upon completion of all medical tests."
Recovery times from vertebral fractures range from three weeks to three months, although Biglia may be able to play with painkilling injections.
The 32-year-old has represented Argentina 57 times, and their first World Cup fixture comes against Iceland on June 16.