Lewan -- Tennesee's 3-time Pro Bowl left tackle -- is facing a 4-game ban after the NFL says he tested positive for ostarine -- a substance that essentially works like anabolic steroids.

Lewan made a video to explain the situation ... and adamantly says that while he did NOT knowingly take the substance -- he is responsible for it getting into his body.

"I want everybody to know that I've never taken this supplement knowingly and I've never cheated the game and I never will."

Lewan says he even took a polygraph test -- which he says he passed (and promised to share later on social media) -- to prove he didn't purposely take the drug.

Taylor then began apologizing to his team, and that's when he got emotional ... breaking down in tears.

"It's just hard," Lewan says ... "I'm sorry to the Tennessee Titans and the fans that I won't be there for 4 games."

ESPN's Adam Schefter reports there's still a chance Lewan could avoid suspension pending the results of further tests ... but Taylor was still apologetic, nonetheless.