Sam Moll's stay with the Pirates' organization lasted three days.
Moll, a 25-year-old left-handed receiver, was claimed off waivers by the Pirates on Monday, but they lost him to the Mariners through waivers on Thursday after trying to avoid using a 40-man on him.
The club now has three open spots on its 40-man roster.
Moll posted a 10.80 ERA in 6 2/3 innings with the Athletics in 2017, and he didn't fare much better in Triple-A.
A third-round pick of the Rockies in 2013, Moll did not reach Triple-A until 2016, when he had a 5.70 ERA in 47 1/3 innings. He started this season there before he was traded to the Athletics in August.
Although the Pacific Coast League is dotted with hitter-friendly parks, Moll still struggled, with a 5.13 ERA and 1.56 WHIP in 47 1/3 innings. He made his major league debut with Oakland in August, but he allowed at least one hit in eight of his 11 outings.
With Moll gone, the Pirates are still searching for a left-handed reliever. They designated Wade LeBlanc and Dan Runzler for assignment earlier this offseason, leaving Felipe Rivero as the only left-handed reliever returning on the 40-man roster. They also claimed Nik Turley, a 28-year-old lefty, off waivers from the Twins earlier this month and chose to keep Turley on the 40-man roster.
However, the Pirates will likely make a move in what is a deep free agent market for relief pitchers. It's possible that Steven Brault, the Pirates' Minor League Pitcher of the Year this season, could move to the bullpen, but they'll also try to add another lefty to complement what could be a young bullpen in 2018.