Montgomery was 4-6 with 4.60 ERA in 16 starts for Seattle. He lasted just 1 1-3 innings against Oakland on Tuesday and was charged with five runs and seven hits.
Mariners manager Lloyd McClendon expects the rookie to work out his command issues in Tacoma as a starter.
Guaipe was 0-3 with a 7.50 ERA in eight appearances for the Mariners in a previous stint. He was 0-4 with a 2.87 ERA in 38 appearances for Tacoma.
Seattle made the moves on Sunday.