Joe Maddon isn't surprised by the Chicago Cubs' recent slump.

"We've lost some games, and things aren't going well right now," the Cubs manager said Thursday prior to a trip to Pittsburgh for a three-game series against the Pirates. "It's just our turn, and we've got to fight through it. Like Winston Churchill once said, when you're going through hell, just keep on going."

The Cubs opened the season with the best start in more than a century, climbing as high as 27 games above .500 and building a 12 1/2-game lead in the National League Central at one point.

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Since June 5, however, the Cubs are 13-18 with seven losses in eight games following a rain-delayed, 4-3, 11-inning loss to the Atlanta Braves on Thursday at Wrigley Field.

"I think there are long-term dividends attached to all of this stuff," Maddon said. "A lot of the young guys getting more at-bats than we had thought at this time in the year -- guys like (catcher Willson) Contreras being forced into a more significant moment in July as opposed to maybe in September. It's just going to benefit him and us."

The Cubs have used five rookies in recent weeks, all of whom joined the team in roughly the last month.

Outfielder and leadoff batter Dexter Fowler, the catalyst to many first half wins, is on the disabled list along with backup catcher David Ross, who was placed on the seven-day concussion disabled list this week.

Cubs starting pitching -- notably the big three of Jake Arrieta, Jon Lester and John Lackey -- have struggled of late, but they look to close the first half with positive performances.

Arrieta (12-3, 2.33 ERA) gets the first crack in the series opener Friday when he faces left-hander Francisco Liriano (5-8, 5.34 ERA). Arrieta is 9-1 with a 1.46 ERA in 12 career starts against Pittsburgh.

Liriano is 0-2 against Chicago this year. He lost a 9-4 decision at Wrigley Field on May 13 and a 6-0 decision to Arrieta on June 17. He is 5-4 with a 3.36 ERA in 13 all-time starts against the Cubs.

Lester goes on Saturday, and he will have Contreras behind the plate after Ross went disabled list. Lackey wraps up the series on Sunday.

Chad Kuhl will start for the Pirates on Saturday, and Jon Niese will pitch Sunday.

The Cubs' division lead is hardly in jeopardy at eight games over the St. Louis Cardinals and 8 1/2 over Pittsburgh. However, the race could tighten further against a surging Pirates team.

Three weeks ago, the Pirates were 15 games behind Chicago following a three-game Cubs sweep at Wrigley Field. Pittsburgh appeared on the verge of slipping out of contention, generating speculation it would write off the season and sell off players to contending teams.

The Pirates instead turned the season around. Pittsburgh is 10-3 since June 24 and had won seven straight until a 5-1 loss to the Cardinals on Thursday at Busch Stadium.

However, even the series-ending defeat was encouraging, as rookie Tyler Glasnow impressed in his big league debut. He pitched 5 1/3 innings and allowed four runs on three hits, struck out five and walked two.

He became the second Pirates pitcher to make his major league debut in three days, joining Steven Brault, who had a four-inning no-decision on Tuesday.

The Cubs won't see either during the series.

Pirates pitching had the lowest ERA (2.64) in the major leagues between June 25 and Wednesday. An improved bullpen has also been a significant element, led by closer Mark Melancon, who has converted each of his past 22 save situations dating back to April 27.

"The bullpen's been incredible, that's tremendous," left-hander Jeff Locke said after he worked four innings Wednesday in the Pirates' 7-5 win over the Cardinals. "To hand the ball over to those guys -- you'd like to go further into the game than that, but every day can't be Christmas."