CHICAGO -- The Cubs have been making do without star shortstop Javier B¨¢ez. The bullpen has also been performing well with Craig Kimbrel sidelined at the moment. Now, Chicago will be tasked with filling the void left by Anthony Rizzo, who is at risk for missing the rest of the regular season.

Prior to Monday's game against the Reds, the Cubs announced that an MRI exam on Rizzo's right ankle revealed a lateral sprain. No surgery is needed, but Rizzo will be in a walking boot for five to seven days. The first baseman hopes to make a comeback before the calendar flips to October.

"I want to play as soon as possible," Rizzo said. "Whether it¡¯s now or Game 1 of the World Series, I want to play as soon as possible."

After the immobilization period, Rizzo will be reassessed and the Cubs will have a better sense of his timeline for return. The first baseman said that he thinks he will have a better sense of where things stand in a few days -- not just by the end of the five to seven days in the boot.

In the meantime, Rizzo will spend his time in the training room doing everything with the team's medical staff to help reach a point where he can test his foot. X-rays taken of Rizzo's ailing ankle on Sunday came back negative for any fractures.

"My body usually responds pretty well, so I'm certainly not ruling it out," Rizzo said of returning before the end of the regular season. "I have every intention of trying to do everything I can with the training staff to get back on the field with the boys. I think in a few days it'll really tell a lot just by how it reacts."

Rizzo injured the ankle while running in to field a bunt single off the bat of Pittsburgh's Trevor Williams in the third inning of Sunday's 16-6 win. The first baseman's ankle buckled before he gloved the ball and got a throw off to first, sending him to the grass in pain after the play.

Once Rizzo was able to shift to his feet, Cubs outfielder Jason Heyward threw an arm around the first baseman's shoulders and helped him to the dugout. Teammate Albert Almora Jr. then helped Rizzo down the steps and to the bench. At one point during the rest of Sunday's game, Kris Bryant headed into the clubhouse to check on Rizzo.

"I do believe that there's galvanizing moments when you do lose key people," Cubs manager Joe Maddon said.

There are also moments to laugh amidst adversity.

Heyward seized on one of those prior to Monday's game, when he saw Rizzo rolling through the clubhouse with his leg supported on a medical scooter.

"It's torture for him, but at the same time," Heyward said with a smile, "we kind of love seeing him roll around. If he's going to make a bunch of jokes about it, we'll make a bunch of jokes about it and have fun with it that way."

In 140 games this season, Rizzo has hit .289 with 26 home runs, 88 runs scored and 93 RBIs. The first baseman's .920 OPS is the second-highest mark during his nine-year career in the Majors. Maddon called Rizzo a "security blanket" for the other infielders and noted that there are things the Cubs can do defensively with Rizzo that other players simply can't pull off at first.

Without Rizzo, though, Maddon said his primary first basemen will likely be Ian Happ and Victor Caratini. The manager noted that catchers Willson Contreras and Jonathan Lucroy could see innings there as well, if necessary. Maddon does not want to move Bryant off third base right now and said Ben Zobrist will not be used at first, either.

"We have different options," Maddon said. "It's going to be one of those daily calls."

Rizzo said the most frustrating aspect of the situation is the timing, given that the Cubs are within striking distance of a postseason spot with fewer than two weeks to play.

"Obviously, these weeks in September and October is why I play baseball," Rizzo said. "It's the most pressure packed, action packed. So, to miss any time right now, it sucks. But, it could be worse. I¡¯m grateful it¡¯s not worse."

That sentiment was echoed by Cubs president of baseball operations Theo Epstein, who would not speculate on Rizzo's timetable for return.

"It certainly wasn't the worst possible news," Epstein said. "I've learned never to rule anything out. Also, there's injuries like this that you just have to give a requisite to let initial healing take place to even have a better idea of what's possible and what's not possible."