Rizzo sprained the ankle while charging in to field a bunt single off the bat of Trevor Williams in the third inning against the Pirates. The Cubs announced that initial X-rays taken of Rizzo's foot revealed no fractures, but the first baseman is scheduled to undergo an MRI exam on Monday to further determine the severity of the injury.
"On the field, he was in some pretty good pain. You knew he was coming out of the game," Cubs manager Joe Maddon said. "We'll wait and see how it plays out. There's the potential that it's not going to be that long. Let's just remain optimistic and see what they say."
Losing Rizzo for any length of time would be a big blow for the Cubs, who are trying to catch the National League Central-leading Cardinals (two games ahead) and fend off the Brewers in the Wild Card race (one game behind Chicago for the second Wild Card).
The Cubs are also without star shortstop Javier Baez (out for the regular season with a hairline fracture in his left thumb) and closer Craig Kimbrel (on the injured list with right elbow inflammation). And there has been a long list of other health woes thread throughout Chicago's turbulent season.
"We've been doing it all year," Cubs third baseman Kris Bryant said. "Anthony's in good spirits, so hopefully we have him back soon. We're going to need him. But I don't know how long it's going to be."
On the play in question on Sunday, Rizzo's right ankle buckled just before he gloved the bunt single by Williams. The first baseman did what he could to get off a throw, but then he dropped to the infield grass in clear pain. Eventually, Rizzo was helped to his feet and then assisted to the dugout by teammate Jason Heyward and a member of Chicago's medical team.
"It was just awkward, the way he twisted and fell," Maddon said. "Had the ball been bunted harder, he would've had a play at third. But the way he checked up, that kind of caused the way he fielded the ball and then the reaction on the throw."
Rizzo, who was replaced at first base by Ian Happ, reached base via walk twice before the injury. Through 139 games this season, the first baseman has hit .289 with 26 home runs, 28 doubles, 93 RBIs, 68 walks and a .918 OPS.
The Pirates went on to score five runs in the third inning after Rizzo's exit, chasing Cubs starter Jose Quintana from the contest in the process. Chicago answered with a five-spot of its own in the bottom of the third to take an 8-5 lead, and kept pouring out runs the rest of the way. Filling in for Rizzo, Happ belted a two-run homer in the fifth.
"We responded well," said Bryant, who had two of the Cubs' five home runs in the win. "Guys stepped up and scored a ton of runs in a really important game."