Richardson suffered a torn lateral collateral ligament and anterior cruciate ligament in his left knee during Sunday's practice.
The six-foot-three, 215-pound slotback is third among the Alouettes this season with 18 catches for 197 receiving yards.
Richardson reached the 1,000-yard receiving mark for a fifth straight season last year.
Richardson was playing in his 10th season in the CFL. The 31-year-old entered the league with Saskatchewan in 2003 and has played with Montreal since 2008.
The Alouettes (2-3) host the Toronto Argonauts (3-2) on Thursday.