According to Stephen J. Nesbitt of the Pittsburgh Post-Gazette, Polanco left the game "after running into the short fence in foul territory along the right-field line. Polanco was carted off the field, holding his right ankle."
After the game, Polanco said X-rays were negative and he has a right ankle sprain, according to Nesbitt, who added he was limping in the clubhouse with a walking boot.
Polanco also hit the disabled list in May with a hamstring injury. The 25-year-old is hitting .265/.340/.417 with six stolen bases.
The Pirates have been without All-Star center fielder Starling Marte since April 18 when he was suspended 80 games for violating Major League Baseball's policy on performance-enhancing drugs.
Losing Polanco only compounds the problems in Pittsburgh's outfield. Andrew McCutchen continues to look like a shell of his former self with a .669 OPS. John Jaso and Jose Osuna will likely be given first crack at holding down right field in Polanco's absence.
The Pirates have struggled already this season, currently sitting in last place in the National League Central, and the loss of Polanco won't make their climb out of that hole any easier.