Diaz was transferred to the 60-day disabled list Thursday, Bill Brink of the Pittsburgh Post-Gazette reports.
Diaz will be on the shelf until mid-July after undergoing elbow surgery earlier in May, so he was an obvious candidate to be transferred to the 60-day DL when a spot on the 40-man roster was needed. That time came Thursday when right-hander Wilfredo Boscan was promoted to the big leagues, so Diaz was shuttled off the 40-man roster for the time being. He'll likely be added back on once he's done with his rehab process.