The shift was required because light-heavyweight Glover Teixeira suffered a knee injury, forcing his bout against former light-heavyweight champion Rashad Evans to be scrapped. It's uncertain when that bout will be rescheduled.
Mir (16-9) has twice possessed the UFC's heavyweight title. He lost it in a rematch with Brock Lesnar in 2009, and has lost two other title shots since. His May 2012 defeat to then-champion Junior Dos Santos has been followed by three other losses.
Silva (18-6) has lost three consecutive fights, starting with his May 2013 defeat to UFC heavyweight champion Cain Velasquez by first-round technical knockout.