The man who claims he was attacked by Oakland Raiders cornerback Sean Smith suffered so much damage to his face, he required major reconstructive surgery, including a metal plate. 

Sources connected to the case tell TMZ Sports ... the victim was diagnosed with 5 facial fractures following the July 4 attack, in which officials say Smith punched and stomped on him in the streets of Pasadena. 

Smith has plead not guilty to felony assault charges -- but we're told the victim is adamant Smith was his attacker. 

We're told the victim had been dating Smith's sister for 2 years -- and have known each other for 6 years. 

As for rumors that Smith was retaliating against the victim for an alleged violent attack on his sister, one source tells us that's "absolutely not true" -- there have never been allegations of domestic violence involving the victim and his GF. 

We're told the victim is extremely traumatized by the attack -- and has withdrawn from school where he was working on his master's degree in social work. 

Smith appeared in court this week where he entered his not guilty plea and his lawyer has said the NFL player will fight the case.