"He's just not coming along great right now," A's manager Bob Melvin said Tuesday. "He's still feeling some tightness in the forearm."
The A's hoped Blackburn would be able to resume a throwing program 10 days after being shut down, but he's now closer to the two-week mark with no progress to show for the extended rest.
Blackburn was 3-1 with a 3.22 ERA in 10 starts as a rookie in 2017, but his season prematurely ended in August after he sustained a deep bone bruise in his pitching hand when struck by a liner in Baltimore.
"You have to wait until he feels good," Melvin said, "but I know he's a little bit frustrated with the way he feels right now."
Melvin said Blackburn underwent a second MRI this week, revealing the same results. There's no structural damage in the pitcher's arm, a relief considering his symptoms -- forearm tightness is often a precursor to Tommy John surgery.
The A's have already lost Jharel Cotton for the season to Tommy John surgery, and their top prospect, hard-throwing lefty A.J. Puk, is expected to undergo the procedure too. The A's pitching depth has subsequently thinned in recent weeks, which led to the signing of a few familiar faces: Trevor Cahill was inked to a one-year deal, while Brett Anderson got a Minor League contract.
Cahill is scheduled to make his first start for Triple-A Nashville on Saturday, with Anderson penciled in for Monday. Cahill is expected to be in consideration for a rotation spot in Oakland by the middle of the month.
Right-handers James Naile and Frankie Montas and left-hander Eric Jokisch will fill out Nashville's rotation.
Jane Lee has covered the A's for MLB.com since 2010.