Rizzo was injured fielding a bunt at first base in the third inning. He had to be helped off the field and could not put pressure on the leg. X-rays taken at Wrigley Field showed no fractures, and an MRI has been scheduled for Monday.
Chicago's postseason pursuit has already been hindered by an injury to shortstop Javier Báez, who is out for the rest of the regular season with a hairline fracture of his left thumb.
The Cubs maintained their one-game lead over Milwaukee for the second NL wild card. The Nationals are 1 1/2 games ahead of Chicago for the first wild card.
Kris Bryant homered twice, and Ian Happ, Kyle Schwarber and Jonathan Lucroy also connected as Chicago battered Pittsburgh for a third straight day. The Cubs scored a total of 47 runs on 49 hits in completing the three-game sweep.
Chicago starter Jose Quintana was pulled after allowing five runs in the third inning, when he fell apart following Rizzo's injury.
After Erik Gonzalez's leadoff double, Pirates starter Trevor Williams bunted toward third base. Rizzo charged aggressively from first and rolled his ankle a few steps before fielding the ball. He threw wide to first base and dropped to the ground in pain.
Brad Wieck (1-1) later replaced Quintana with two on and struck out both batters he faced to end the inning. The Cubs rallied for five runs in the bottom of the inning to lead 8-5, starting with Bryant's second homer of the game and ending when reliever Michael Feliz threw wildly on an appeal play at third base.
Williams (7-7) allowed seven runs in 2 1/3 innings. Pablo Reyes drove in three runs for the Pirates.
Bryant gave the Cubs an early lead with a three-run blast in the first.
Happ, who replaced Rizzo, hit a two-run shot in the fifth to make it 12-6. Jason Heyward drove in a run in the sixth and Schwarber hit a two-run shot in the seventh. Lucroy capped the scoring with a solo homer in the eighth.
Pirates: 1B Josh Bell (groin strain) missed his second straight game. He is listed as day-to-day after exiting in the fifth inning on Friday. ... OF Starling Marte (left wrist sprain) didn't start for the seventh straight game.
Cubs: Closer Craig Kimbrel (right elbow inflammation) threw 20 pitches in a bullpen session before the game. He reported no issues, but the team will see how he feels Monday before deciding on the next step. ... INF Addison Russell was placed on the seven-day concussion list Sunday retroactive to Thursday. He was hit in the head by a pitch last Sunday at Milwaukee.
Pirates: RHP Mitch Keller (1-4, 8.29 ERA) faces LHP Marco Gonzales (15-11, 4.30) Tuesday night in the opener of a three-game home series against Seattle.
Cubs: LHP Cole Hamels (7-7, 3.89 ERA) starts Monday night in the opener of a three-game series against the Reds. RHP Sonny Gray (10-7, 2.80) pitches for Cincinnati.
More AP MLB: https://apnews.com/MLB and https://twitter.com/AP_Sports
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