Play video content EXCLUSIVE 2/20/18 TMZSports.com

UFC legend Anderson Silva is still adamant he never knowingly used steroids -- but says if the positive test results are accurate, the only explanation is "contaminated supplements."

Silva tested positive for two banned substances in a U.S. Anti-Doping Agency drug test conducted back in October. Silva is now facing a maximum 4-year suspension from fighting.

But, "The Spider" tells TMZ Sports he would NEVER knowingly juice up -- saying it would be a "stupid" thing to do at the tail end of his career.

His explanation -- "Maybe the supplements I'm using are contaminated."

Silva says his lawyers are still fighting to clear his name -- and he's confident he'll fight again.

In fact, he's still focused on getting a fight with boxing legend Roy Jones Jr.

And get this ... if 42-year-old Silva IS banned for 4 years -- he does NOT plan on retiring ... insisting he'll still take a fight even in his late 40s because he loves the sport so much.