UFC officials announced the move on Friday.
Rivera signing on to fight Mizugaki is the latest piece in the puzzle that the UFC 173 fight card has become.
Original main eventer Chris Weidman earlier this week dropped out due to injury, after already having his opponent altered. The UFC then opted for bantamweight champion Renan Barão to defend his belt against T.J. Dillashaw in the fight card’s new headliner.
Dillashaw, however, had already been slated to face Mizugaki, so that left the Japanese fighter without an opponent.
Less than 24 hours after tapping Dillashaw for the main event, UFC officials enlisted Rivera to step in against Mizugaki.
The UFC on Friday also added another high profile bout to the UFC 173 fight card.
When Tarec Saffiedine opted out of his UFC 172 bout with Jake Ellenberger due to a training injury, Robbie Lawler agreed to take his place. Having just fought at UFC 171 a couple weeks ago, UFC officials slotted the new Lawler vs. Ellenberger match-up for the UFC 173 fight card on Memorial Day weekend.