Jon Lester surrendered 10 runs in the first inning of the shortest start of his 12-year career, and the Cubs stumbled to a 14-3 loss to the Pittsburgh Pirates.
Chicago (43-45) heads into the break with a losing record for the first time since it was 40-54 in 2014. The reigning World Series champions also trail NL Central-leading Milwaukee by 5 1/2 games after the Brewers held on for a 5-3 victory against the New York Yankees.
The Cubs were 53-35 at the break a year ago.
Francisco Cervelli matched a career high with five RBIs and Jordy Mercer drove in three runs as the Pirates closed out a 5-2 road trip with their fifth win in six games. All-Star Josh Harrison homered in the ninth and Chad Kuhl pitched three innings of one-run ball in an emergency start after Jameson Taillon was scratched due to flu-like symptoms.
Pittsburgh sent 15 batters to the plate in its highest-scoring inning since it got 10 in the seventh against Colorado on May 17, 2009. It was the first time it scored at least 10 in the first inning since June 8, 1989, against Philadelphia.
Cervelli hit his third career grand slam into the basket in left with two out. McCutchen followed with a drive to left-center for his 17th homer.
Lester (5-6) was charged with four earned runs and six hits. He also struggled in his previous outing, allowing six runs in five innings in a loss to Tampa Bay on Tuesday.
Kuhl allowed two hits and walked three on just two days' rest after he pitched a career-high seven innings in a 6-3 win at Philadelphia on Thursday night. A.J. Schugel (1-0), the second of three Pittsburgh relievers, pitched three innings for the win.
The Pirates (42-47) improved to 5-1 at Wrigley this year and stayed seven back of Milwaukee. They were 46-43 at the All-Star break last season.
Cubs rookie Victor Caratini entered before the Pirates batted in the fifth and went 3 for 3 with two doubles. He snapped a 0-for-9 skid to begin his career.
Mike Montgomery (1-6, 3.75 ERA) will start Friday night's game at Baltimore. Montgomery replaced Lester and allowed two runs and three hits in three innings.