Diehl was optioned to Triple-A Albuquerque on Thursday.
He was only up with the Rockies for a few days before heading back to the minors. Mike Dunn (forearm) was activated from the injured list in a corresponding move.
Diehl had his contract purchased from Triple-A Albuquerque on Monday, Thomas Harding of MLB.com reports.
Diehl will replace Seunghwan Oh (abdomen) -- who was sent to the injured list in a corresponding move -- on the active roster and in the bullpen. The southpaw, who was acquired from the Yankees prior to the season, compiled a combined 2.30 ERA, 0.88 WHIP and 29:8 K:BB in 27.1 innings between Albuquerque and Double-A Hartford before earning a promotion.
Diehl was sent from the Yankees to the Rockies on Saturday in exchange for Mike Tauchman.
Diehl reached Double-A for the first time last season, finishing with a 1.35 ERA in 26.2 innings. Over the course of his minor-league career, the-year-old has an impressive 36.2 percent strikeout rate.
Diehl (back) is listed among the Yankees' available pitchers for Wednesday's Grapefruit League game against the Cardinals, Randy Miller of The Newark Star-Ledger reports.
Diehl's last spring appearance came Saturday against the Pirates and ended prematurely after he got drilled in the back by a line drive. His availability just a few days later suggests his removal from that contest was precautionary and that he avoided any major injury. The 24-year-old lefty is likely to open the 2019 campaign at Triple-A Scranton/Wilkes-Barre or Double-A Trenton.
Diehl left Saturday's game against the Pirates after being hit on the back by a line drive, Kristie Ackert of the New York Daily News reports.
Diehl was the victim of a hard-hit ball by Jason Martin. He walked off the mound and seemed to be mostly fine as he headed back to the clubhouse with a team trainer, but the Yankees had little reason to take a chance with him in a spring training game.
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