With just two weeks to go before the event, Riddle, of TUF 7 fame, injured his knee and was forced withdraw. Sources close to the fighter have confirmed that Riddle will require surgery on the knee which will keep the young fighter out for the next six to eight weeks.

Brown, Riddle's fellow TUF 7 alum, is coming off of a very good showing in a narrow decision loss to Dong Hyun Kim.

Below is a look at the updated UFC 91 fight card featuring the return of Randy Couture:

Main Card:

Randy Couture vs. Brock Lesnar

Kenny Florian vs. Joe Stevenson

Nick Catone vs. Amir Sadollah

Gabriel Gonzaga vs. Josh Hendricks

Demian Maia vs. Nate Quarry

Preliminary Card:

Dustin Hazelett vs. Tamdan McCrory

Jorge Gurgel vs. Aaron Riley

Jeremy Stephens vs. Rafael dos Anjos

Mark Bocek vs. Alvin Robinson

Matthew Riddle vs. Ryan Thomas