The Mariners designated Muckenhirn for assignment Wednesday.

The southpaw will move off the Mariners' 40-man roster with Seattle needing to clear a spot for right-hander Emerson Hancock, whose contract was selected from Double-A Arkansas ahead of his first MLB start Wednesday versus the Padres. Since being acquired in a trade with the Mets on July 3, Muckenhirn has pitched exclusively at Triple-A Tacoma, where he turned in a 9.35 ERA and 1.95 WHIP over 8.2 innings.