Yagin withdrew from the bout on Tuesday after suffering a head injury in training. Phan quickly asked for the short-notice fight via his Twitter feed. UFC officials announced the matchup Wednesday.
Phan (18-10) is coming off a split-decision victory over Cole Miller on a UFC on Fox card in August. The California resident is 2-3 since joining the UFC in 2010, following an appearance on "The Ultimate Fighter" reality series.
Siver (20-8) will look to continue his success at featherweight after dropping from the 155-pound division earlier this year. ESPN.com already ranks the 33-year-old the No. 6 fighter in the division.
He posted a unanimous decision win over Diego Nunes at a UFC on Fuel card in April.