Panathinaikos say defender Juankar was "taken to hospital for tests" after being hit by a firecracker during the club's Greek Superleague match at Olympiakos on Sunday.
The Spanish substitute collapsed on the edge of the field after being struck while warming up before being carried off on a stretcher.
The match was called off afterwards with the score 1-1 after 50 minutes.
Panathinaikos say the result of the game will be "decided in the courts".
Juankar, 33, who plays primarily at left-back, has made 109 appearances for the Greens since signing from Malaga in 2020.
The Athens derby, referred to as the derby of the eternal enemies, has been marred by ugly scenes in the past, with a 2019 match between the rivals abandoned when fans fought with riot police.
Olympiakos host Premier League side West Ham in a Europa League group stage match on Thursday.
