Since the deal was finalized before Sept. 1, Donaldson is eligible to play in the post-season.
Merryweather, the player to be named later in the trade, was 7-9 with a 5.32 earned-run average in 2017 over 25 games for triple-A Columbus and double-A Akron.
The 26-year-old native of Berkeley, Calif., missed this past season after undergoing Tommy John surgery in March.
Merryweather is 23-20 with a 3.92 ERA over four minor-league seasons.
The Blue Jays designated utilityman Jon Berti for assignment to make room on the 40-man roster.