Volkmann, who announced the move and bout on his Twitter, is making the weight cut after two losses as a welterweight in the UFC against Paulo Thiago and Martin Kampmann.
Volkmann (9-2) had entered the UFC with a flawless 9-0 record before incurring the back-to-back losses. Volkmann trains out of Greg Nelson's Minnesota Martial Arts Academy.
Torres (14-2) of Nova Uniao is coming off a UFC debut loss to Melvin Guillard by unanimous decision earlier this month at UFC 109 that could have gone either way. Torres carried an eight-fight win streak into his Octagon debut.
UFC Fight Night 21 is headlined by Kenny Florian vs. Takanori Gomi and Roy Nelson vs. Stefan Struve.