Police reported that several dozen fans of holders Beroe invaded the pitch on the eve of the Bulgarian Cup last-16 second leg against Ludogorets, throwing missiles, stones and other objects.
Dutch midfielder Mitchell Burgzorg was hurt after being hit on the head by a stone, assistant coach Yasen Valchinov was struck on the calf by a firework while a masseur sustained a minor injury.
Burgzorg, who is also a rapper under the name of 'Priester', was taken to hospital in the southern town of Stara Zagora.
"We're still searching for the perpetrators," a police spokeswoman told Reuters. "Police reacted very quickly and prevented more serious incidents."
Beroe, the 1986 Bulgarian league champions, hit out at the violence.
"The management condemns any hooliganism and cannot tolerate such actions," the club said in a statement. "We will not allow such an escalation of tension to happen during the game on Sunday."
Ludogorets chief executive Angel Petrichev praised the efforts of the police.
"Fortunately there were police and security guards around the stadium and they prevented something much worse," he said. "I would like to thank the police in Stara Zagora for responding so quickly."
Ludogorets won the first leg 2-1 on Thursday.
In another example of crowd violence, police arrested 18 hooligans during and after Saturday's 'eternal derby' between CSKA Sofia and city rivals Levski.
The tense Bulgarian Cup encounter ended 0-0 with Sofia police saying that most of the troublemakers were arrested for drunken behaviour or public order offences.
The domestic championship has been plagued by crowd trouble in recent years.
Last month coach Ivaylo Petev left Levski one day after being forced out of his first news conference by hostile fans of the 26-times Bulgarian champions.
- Crime & Justice
- Sports & Recreation
- Mitchell Burgzorg