Per NFL Network's Ian Rapoport, Manziel will start against the Hamilton Tiger-Cats.
The Tiger-Cats signed Manziel in May but traded him to Montreal on July 22, as he was stuck behind Jeremiah Masoli on the depth chart.
"This is a move both (Alouettes GM) Kavis Reed and I felt we needed to make at this time," Montreal head coach Mike Sherman said about the deal. "Neither one of us believes in the status quo and felt we needed to shake things up. Johnny is someone we believe can elevate one of the most important positions on our team."
Manziel redshirted at Texas A&M during the 2011 season, Sherman's final year as head coach of the Aggies.
The Alouettes are 1-5 through six games and tied with the Toronto Argonauts for the CFL's worst record. Their 92 points scored are the league's fewest.