CHICAGO -- Alec Mills did not know he was starting when he arrived at Wrigley Field on Friday afternoon. A few hours before the Cubs’ game against the Brewers, the righty learned it was a possibility and immediately went from a day off to diving into scouting reports.

“The way he handled that,” Cubs manager David Ross said, “it just really felt like a veteran-type pitcher tonight. He did a great job in handling that adversity.”

Mills made a handful of mistakes that contributed to a 4-3 loss to the Brewers, but the right-hander’s six-inning effort helped save Chicago’s staff with seven games scheduled in the next five days. And now the Cubs are working through another complication with righty Tyler Chatwood, who was scratched from Friday’s planned outing.

• Box score

Chatwood developed some mid-back tightness prior to the game, forcing Mills into action for the Cubs, who slipped to 13-4 on the year. Now, Ross said right-hander Colin Rea is slated to move out of the bullpen to start Saturday’s 2:20 p.m. CT tilt, with veteran Jon Lester on deck for Sunday’s series finale against Milwaukee.

And then?

“See how Tyler checks out, see if Monday's an option,” Ross said.

That is when the Cubs will welcome the Cardinals to town for five games in three days, including doubleheaders on Monday and Wednesday (with St. Louis as the home team in Game 2 each time). Rea might have been an option to start one of those twin bills, but now Chicago’s decision-makers will be weighing alternatives. Adbert Alzolay, Tyson Miller and Justin Steele could be among the possibilities.

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If Chatwood is recovered enough to start on Monday, then he would presumably be paired up with righty Kyle Hendricks. The typical rotation order would then call for Yu Darvish to take the ball on Tuesday. Mills could stay on a five-day routine and start again on Wednesday, with another starter likely coming from the South Bend (Ind.) alternate training site.

How Chatwood feels on Saturday will impact those conversations in the coming days. In the meantime, Ross was thankful that Mills -- whose last start was on Aug. 3, given that last weekend’s series in St. Louis was postponed -- was ready and willing to step in against the Brewers.

“It's not too often that you can scratch a starter and one of your other ones are ready to go,” Ross said. “And Millsy having just such an ease about the way he competes and his personality, it just made it really easy to pull that trigger without a ton of preparation for him. He started studying the scouting report, played catch and then he got in his routine.”

Mills was sharp for the first four innings, going toe to toe with Brewers right-hander Brandon Woodruff. They each held the opposing lineup without a hit leading up to the fifth, when the lineups began breaking through in their second looks against each starter.

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The Brewers struck for one run off Mills in the fifth, but Chicago’s offense answered by chasing Woodruff with a three-run, 32-pitch rally off the righty. Overall, the Cubs sent nine batters to the plate in the home half of the fifth, forcing Milwaukee’s staff to toil through 40 pitches.

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Milwaukee undid all that work in a span of three pitches.

“This one's frustrating, for as well as I thought I pitched tonight,” Mills said.

In the sixth, Mills allowed a single to Eric Sogard, hit Keston Hiura with the next offering and then surrendered a first-pitch, three-run homer on a hanging changeup to slugger Christian Yelich. That gave the Brewers a 4-3 lead against Mills, who struck out seven, walked one and logged six innings in his emergency start.

“Just made a mistake to the best hitter in the lineup,” Mills said. “I mean, he's an MVP. You can't let that happen. It's a mistake, and I'll learn from it.”

More important for the Cubs, Mills has learned how to be adaptable.

Coming into this season, Mills was expected to be in the bullpen, but an injury to José Quintana forced the righty into the fifth spot on Opening Day. Through three outings, he has turned in a 2.84 ERA with 14 strikeouts, 10 hits allowed and six walks in 19 innings. Mills perplexes more than he overpowers, and he has looked right at home in the rotation.

“I think I've preached to myself and I've said it plenty of times to you guys,” Mills said, “I'm just here to do whatever I can to help. I think early on in the season it was to fill in that fifth spot for Q, and tonight it was to throw for Chatwood when I had extra days of rest.

“I had no problem going to that. I was excited to throw.”

It did not net a win, but the Cubs’ staff is better positioned for the next several days.

“Anything we throw at this guy, he takes it with a smile,” Ross said.