Polanco's ankle got caught awkwardly in the railing separating the stands from the playing field of PNC Park while he was chasing after a foul ball.
X-rays were negative and Polanco will see a doctor before getting an update Tuesday, the team announced after the game.
Jose Osuna replaced Polanco in right field.
Polanco was 1-for-2 in Monday's game before exiting with the injury. He is batting .269 with three home runs and 11 RBIs this season.
He was just activated from the disabled list last Thursday after being sidelined for 10 games with a left hamstring injury.
Information from The Associated Press was used in this report.