Losing Freeman clears a space on the Mariners’ 40-man roster because he had been on an optional assignment at Triple-A Tacoma.
Heston was designated for assignment Wednesday in order to clear roster space for outfielder Andrew Aplin, whom the Mariners acquired in a waiver claim from Houston.
The Mariners have one opening on their 40-man roster.
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Freeman, 29, split time this season between the Mariners and Tacoma. He had just two hits in 30 big-league at-bats (an .067 average) in 16 games but batted .356 (21-for-59) in 15 games for the Rainiers.
The Mariners acquired Freeman in an Aug. 1, 2016 waiver claim from Arizona and retained him on a minor-league contract in March when he cleared waivers after being designated for assignment.
Heston, 29, was hit hard in two big-league outings, allowing 12 earned runs in five innings, but was 2-1 with a 3.41 ERA in six starts at Tacoma.
The Mariners acquired Heston on Dec. 7, 2016 in a trade from San Francisco for a player to be named later.
Bob Dutton: @TNT_Mariners