Sporting Lisbon's Netherlands global Bas Dost was injured after around 50 hooded supporters, armed with sticks and belts, attacked players and staff at the club's training complex, Portuguese media reported on Tuesday.
Sporting issued a statement condemning what it calls "acts of vandalism and the attacks on (our) athletes, coaches and professional staff".
Sporting Lisbon missed out on Champions League football under agonising circumstances on the last day of the Primeira Liga season.
The assailants covered their faces to avoid being identified and some players suffered injuries during the incident, with photos emerging of Dost suffering what appeared to be severe cuts to his head.
Sporting Lisbon have confirmed that a number of its players and staff were assaulted by a group of fans who forced their way into the club's training ground on Tuesday.
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RTP showed an image of bloody wounds to the head of Sporting's Dutch striker Bas Dost and of a changing room that had been trashed. Tuesday's events were hardly ideal preparation for their upcoming Portuguese Cup final against Aves on Sunday evening.
"This is not and can not be Sporting", it added.
Players have this season been frequently criticised by chairman Bruno do Carvalho, who at one stage suspended 19 of them. "We will take all steps to establish responsibility for what has happened, and we demand the punishment of those who have acted in this absolutely regrettable manner".