McCullers was placed on the 10-day disabled list Monday due to lower-back discomfort, Jake Kaplan of the Houston Chronicle reports.
McCullers just cruised through a strong outing against the Royals on Thursday, giving up only one run on two hits while striking out eight. It sounds as though the back issue first surfaced when he tossed a bullpen session in advance of his next scheduled start Tuesday against the Rangers, prompting the Astros to exercise caution with the right-hander. According to Kaplan, the Astros aren't bracing for a long absence from the up-and-coming ace, who may only miss one turn, but it's nonetheless another blow to a rotation that's already had four other starters hit the DL this season. The Astros haven't named a replacement in the rotation for McCullers, but prospect Francis Martes -- who tossed 3.2 innings in relief Friday against the Angels -- could be called upon to make a spot start on three days' rest.