The Pirates optioned Cook to Triple-A Indianapolis on Thursday.
Cook will move to the minors in order to clear a spot on the active roster for top prospect Konnor Griffin, who is slated to make his MLB debut against the Orioles on Friday. The 27-year-old Cook slashed .245/.321/.379 across 380 plate appearances at Indianapolis last year, and he'd be a candidate to return to Pittsburgh if one of the big club's outfielders go down with an injury.