MMA Junkie revealed the bout earlier today.
Stephens was defeated by Din Thomas in his UFC debut at UFC 71 but won his next three-straight bouts including UFC victories over Diego Saraiva at UFC 76 and Cole Miller at UFC Fight Night 12. In his latest bout Stephens suffered his second octagon loss to Spencer Fisher via unanimous decision at The Ultimate Fighter 7 Finale in June.
Dos Anjos has won nine-straight fights including a first-round victory over Takaichi Hirayama at a Hyne Pancrase event and a split decision win over Takafumi Otsuka at Fury Fighting Championship 6 in July. Over half of his wins have come via submission.
UFC 91 features the return of UFC heavyweight champion Randy Couture against challenger Brock Lesnar and also includes a key lightweight matchup between Kenny Florian and Joe Stevenson.