In his five starts for Louisville, the 22-year-old Holmberg is 0-4 with 10.50 ERA, nine walks and 13 strikeouts in 18 innings.
"We haven't seen the best of David yet but we've certainly seen it on video and in Arizona," Reds manager Bryan Price said. "[Pitching coach] Jeff Pico was fully encouraged he's going to be an impact guy here for us at some point."
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.