MILWAUKEE -- It was only two days ago that the Cubs thought they finally had their lineup healthy and whole for the rest of the season. Ben Zobrist and Willson Contreras were back, and both Kris Bryant and Javier Báez were in the announced order after health issues.

And then Baez took batting practice at Miller Park, where pain in his left hand persisted. And then the All-Star shortstop headed to Chicago, where an MRI exam on Saturday morning revealed a hairline fracture in his left thumb. And now the Cubs are faced with the very real possibility of not having Baez for the remainder of the regular season.

"It's really unfortunate. Javy's the heart of this team," Contreras said prior to Saturday night's game against the Brewers. "We're going to miss him a lot, but like somebody said, 'One steps out, one steps in.' Of course, we're going to miss him and hopefully he recovers really soon."

The next step for Baez will arrive Monday, when he will see the Cubs' hand specialist to discuss a course of action and establish a timetable for return. In the meantime, Chicago will lean on Addison Russell as its regular shortstop, making David Bote and Zobrist the backup options until Baez is deemed ready to return.

Baez, 26, initially injured the thumb during a stolen base against the Brewers in the third inning of Sunday's game at Wrigley Field. The shortstop remained in the game until the sixth inning and then underwent X-rays, which came back negative for anything major. Baez sat out Monday and Tuesday, and then the Cubs had an off-day Wednesday. Manager Joe Maddon called it an "All-Star break" for Baez.

On Thursday, Baez sat down with Maddon at Miller Park and expressed that he felt good enough to play, so the manager slotted him in the starting lineup. Things changed within the next few hours, when Baez reported still feeling pain while hitting.

"I figured something was up," Maddon said. "I mean, he wants to play and he comes out to the ballpark wanting to play, and then he swings a little bit and it's hurting. So you know, he's not going to mess around. I was concerned something like this may be the result. And now we just have to wait until he sees the specialist and then figure it out from there."

In one sense, Baez's statistics show what he means to the Cubs.

The shortstop has hit .281/.316/.532 with 29 home runs, 38 doubles, 85 RBIs and 88 runs scored in 133 games this season. He started for the National League All-Star team for the second year in a row and was the NL's runner-up for MVP last season. In the field, he has piled up 15 Defensive Runs Saved at shortstop and posted some of the top defensive metrics across the board at the position.

The numbers alone do not capture Baez's importance, though.

"He does amazing things and he makes plays and he energizes us on the field," first baseman Anthony Rizzo said.

"He plays for us like with his hair on fire," Contreras added.

"It's hard losing a guy of his charisma also," Maddon said. "There's all that. The things that he does to pick us up when things aren't going well -- the stolen base, the extraordinary slide, the [big] play, the home run -- the things that Javy does, we're just going to have to find that in other areas."

The Cubs have been trying to do that to a certain degree all season.

Closer Craig Kimbrel (right elbow) landed on the injured list on Thursday. Contreras missed most of August with a hamstring injury. Bryant has battled a nagging knee issue since late June. Rizzo has missed games due to his back. Yu Darvish, Jon Lester, Kyle Hendricks and Cole Hamels have each had injury setbacks. Zobrist missed four months while on the restricted list.

The list goes on, too.

"That's the season for anyone," Rizzo said with a shrug. "You play baseball. You have guys come in and out and up and down, and streaks and cold streaks. As professionals, we have to step up. Obviously, you're [losing] an All-Star on and off the field, perennial MVP candidate, so it's not easy.

"But, we can't just hang our heads. We've got to play."