Thiago Silva is officially OFF THE BILL for "UFC 171" in March ... after allegedly threatening to kill his wife ... and he's been replaced by a Ukranian bad ass, TMZ Sports has learned.
After declaring Silva will never fight in the UFC ever again, Dana White tells TMZ Sports ... 21-year-old phenom Nikita Krylov will fill Silva's spot and fight Ovince St. Preux at UFC 171 in Dallas.
We're told Krylov -- a former heavyweight -- will drop down to the light heavyweight division for the fight.
White is stoked about the matchup, especially after the way Krylov -- who calls himself the Al Capone of MMA -- knocked out Walt Harris with a head kick just :25 seconds into the fight at "UFC on FOX 10."
Sorry Thiago ... but that's what happens when you threaten to shoot people with a gun (allegedly).