Thiago Silva -- Officially Replaced in 'UFC 171' ... After Insane Gun Arrest

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 and he's been replaced by a Ukranian bad ass, TMZ Sports has learned.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."