Mengden cleared waivers and was outrighted to Triple-A Omaha on Sunday.
Mengden was designated for assignment Friday, and he'll now head back to Omaha after a one-day stint in the majors. He allowed three runs on five hits and a walk with two strikeouts in a loss versus the White Sox on Thursday. Without a spot on the 40-man roster, it's unlikely Mengden plays much of a role with the big club over the last month of the season.