Couture is slated to meet Ultimate Fighter 15 finalist Al Iaquinta in a lightweight bout in Milwaukee on Aug. 31. The news was first reported by the Las Vegas Sun.
Mulhern announced his fight pullout over Twitter on Tuesday, posting a picture of his fractured hand. In a follow-up to a fan's question, Mulhern attributed the injury to a "bad overhand right."
Couture (6-2), the son of five-time UFC champion Randy Couture, will be making his second Octagon appearance. He lost a second-round TKO to Ross Pearson on April 6 in Stockholm in his UFC debut, which snapped Couture's four-fight win streak.
Iaquinta, who trains with the Serra-Longo Fight Club, holds a record of 5-2-1. His last bout was a submission loss to Michael Cheisa in the TUF 15 championship fight on June 1 of last year.