A replacement has yet to be announced, although the promotion has targeted Brazilian lightweight Rafael Dos Anjos as a possible opponent, sources say.
Sotiriopoulos (14-3) is looking to bounce back from his first UFC loss, a unanimous decision against Dennis Siver in February in his home country of Australia. Prior to the loss, he had posted seven straight wins in the lightweight division, including four finishes.
For Dos Anjos (14-5), it will be his first time in the cage since suffering a jaw injury in the third round of a loss to Clay Guida at UFC 117 last August. Undefeated before starting his UFC career, Dos Anjos is 3-3 in the promotion since 2008. The loss to Guida snapped a three-fight win streak.
It's a disappointing setback for Dunham (11-2), who is still considered a bright 155-pound prospect despite going 0-2 in his last two fights. After winning his first 11 professional fights, Dunham dropped a controversial split decision to Sean Sherk last September before taking a first-round TKO loss to Melvin Guillard in January.
Brett Okamoto covers mixed martial arts for ESPN.com.