8:20 AM PT -- A source extremely close to Fuller tells us the football star had gone to physical therapy to rehab his knee in the hours before his death ... and it was clear something was wrong.
We're told Fuller tore his ACL playing in a flag football game roughly 2 weeks ago and had surgery to repair the injury at the end of September.
Fuller went to physical therapy for the first time on Wednesday (10/3) as part of his recovery and completed a full session without issue.
When he arrived back to his home, we're told he "went to the ground" and asked for water -- clearly feeling something was wrong.