Harrison will start Thursday's game in Pittsburgh, Todd Rosiak of the Milwaukee Journal Sentinel reports.
Harrison's last appearance came in relief, and he was charged with four runs without recording an out against the Reds. In his last five starts, the left-hander has logged a 6.95 ERA, 2.23 WHIP and 23:12 K:BB over 22 innings. Given those struggles, Harrison will not be guaranteed any more starts beyond Thursday.