Diehl cleared waivers and was outrighted to Triple-A Louisville on Thursday, Mark Sheldon of MLB.com reports.
Diehl was designated for assignment by Cincinnati on Monday, but he'll remain in the organization after losing his spot on the 40-man roster. The 27-year-old has posted a 2.73 ERA and 0.91 WHIP in 62.2 innings over 66 appearances in Louisville over the last two seasons.