The Italy international was carried off on a stretcher after appearing to collide with a player following a corner and falling awkwardly.
A team-mate reportedly reacted to make sure Antonelli, 29, did not swallow his tongue.
Milan have yet to comment on Antonelli's condition.
AC Milan defender Luca Antonelli was taken to hospital with a suspected head injury during the 1-0 Serie A defeat by Udinese at the San Siro.