The 23-year-old from Lethbridge, Alta., is coming off a first-round TKO win over former UFC fighter Forrest Petz on a Score Fighting Series card in Hamilton. Prior to that, Mein (26-8) went 2-1 in Strikeforce.
Miller (14-6 with one no contest) dropped down to welterweight last time out, submitting Ricardo Funch last June. He was 5-5 in the UFC as a middleweight.
The 31-year-old New Jersey native is the older brother of lightweight Jim Miller.
The main event of UFC 158 is a welterweight title defence between champion Georges St-Pierre and Nick Diaz.
The Bell Centre card also features Canadian welterweight Rory (Ares) MacDonald versus Carlos (Natural Born Killer) Condit, and 170-pound contender Johny Hendricks versus Jake Ellenberger.
Montreal's Patrick Cote drops down to welterweight to face Bobby Voelker.