He first thought it was consensual, but then he saw the woman was unconscious - and now, the university footballer is being hailed for helping stop an alleged sexual assault.
"I mean, at that point, I'm like, this isn't right," Garcia told WFOR-TV.
"This girl is probably drunk, there's no possible way she could've given consent."
Garcia said the man's friends were watching, and did nothing to help. The incident was videotaped.
"I grabbed the guy by the shoulder and said, 'Get off of her, this isn't right,' and the guy yelled at me like, 'Mind your own business,'" Garcia said.
Police later arrested Christopher Shaw, 34, and charged him with sexual battery of the 19-year-old woman.
Gainesville police spokesman Ben Tobias confirmed that Garcia and Leroyea Simmons helped stop the incident and identified Shaw in a photo line-up.
According to the Sun Sentinel, the police report states the woman was "unable to walk on her own and had difficulty talking and keeping her eyes open."
"Police say she told investigators in a hospital interview that she didn't know Shaw and that she tried to push him away but was physically helpless because she was going in and out of consciousness due to her level of intoxication.
"Upon viewing the videos, it is clear the victim was mentally and physically unable to give consent due to her level of intoxication.
"The victim was slumping over and unable to hold up her head. The (defendant) can be seen pushing the victim back up. ... When pushed back up the victim's eyes were closed, and her head fell back against the dumpster."
According to Shaw, the woman initiated the contact by pulling him behind the dumpster as he was walking though the alley.
He denied having sexual contact with the woman, besides a harmless kiss.
Police say video evidence "completely contradicts" Shaw's testimony.
According to Garcia, Shaw's friends "were there on the other side of the dumpster. Maybe they didn't see it, I can't say 100 per cent, but they were there."
Garcia's intervention came after a recent spate of sexual assault cases nationally involving student-athletes.
Last month, a judge sentenced former Stanford University swimmer Brock Turner to six months in jail and ordered him to register as a sex offender for life after a jury convicted him of raping a student behind a campus dumpster in January 2015. Prosecutors had sought a six-year sentence.
A jury also convicted former American college football player Brandon Vandenburg guilty on multiple counts of aggravated rape and aggravated sexual battery after encouraging his teammates to rape an unconscious woman he had been dating.
Thanks to Cristian Garcia for stepping up when a helpless victim needed his help. https://t.co/P1F5i3eANb— Gainesville Police (@GainesvillePD) July 22, 2016