Polanco, 22, hit .241/.308/.349 (86 OPS+) with nine doubles, six home runs and 12 stolen bases in 64 games after being called up in June. He is currently mired in a 1-for-30 (.033) slump, hence the demotion.
The emergence of Travis Snider -- the ex-Blue Jays outfielder is hitting .268/.336/.435 (116 OPS+) overall and .319/.366/.585 (170 OPS+) in the second half -- robbed Polanco of at-bats and made sending him down nice and easy.
There was quite a bit of ruckus when the Pirates opted to keep Polanco in Triple-A earlier this year even though the offense struggled. He was presumably kept down for service time reasons, delaying his free agency.
Before being called up, Polanco hit .347/.405/.540 with 17 doubles, seven homers and 15 steals in 62 Triple-A games. Baseball America ranked him as the 10th best prospect in baseball before the season.
The Pirates are currently 67-63 overall, five games back in the NL Central and 1.5 games back of the second NL wild-card spot.
Rosters expand in seven days, so Polanco's demotion to Triple-A figures to be short-lived. The team has not yet announced the corresponding roster move.