The Pirates designated left-hander Aaron Thompson for assignment to create 40-man roster space for Chase d'Arnaud, according to MLB.com's Jenifer Langosch (on Twitter). Pittsburgh optioned Josh Harrison to Triple-A yesterday to create room on the active roster.
In 67 innings at Double-A Altoona, Thompson has a 4.97 ERA with 5.8 K/9 and 2.4 BB/9. The 2005 first round pick has posted a 4.37 ERA with 6.8 K/9 and 3.1 BB/9 in 747 1/3 innings as a minor leaguer. D'Arnaud, 24, has a .280/.347/.418 line at Triple-A this year and is set to debut tonight as Pittsburgh's third baseman.