Triple-A Indianapolis placed De Los Santos on its 7-day injured list Aug. 19 with an unspecified injury and transferred him to the 60-day IL on Aug. 28.
Though the nature of De Los Santos' injury is uncertain, the Pirates have determined that it's significant enough to keep the right-hander on the shelf for the remainder of the season. The 25-year-old turned in a 3.33 ERA, 1.23 WHIP and 18:13 K:BB in 24.1 innings over 22 appearances with Pittsburgh earlier this season before being optioned to Triple-A on Aug. 8 and suffering the injury shortly thereafter. De Los Santos retains a spot on the Pirates' 40-man roster, but depending on the severity of his injury, he could be a non-tender candidate this winter.