The Chicago Cubs aren’t going to panic. Not over their suddenly struggling starting staff, and not over their dwindling division lead, either.

Josh Bell’s grand slam in his second major-league at bat was one of four homers by the Pittsburgh Pirates in a 12-6 win over Chicago on Saturday night.

The Pirates have won nine of 10 over a stretch coinciding with the Cubs losing nine of 10, meaning Chicago’s lead in the National League Central is down to 6 1/2 games.

“I feel like we’re fortunate to be (up 6 1/2 games) right now, actually,” manager Joe Maddon said. “I don’t believe it’s a negative; I think it’s a positive.”

Jordy Mercer followed Bell’s slam with a home run of his own off of Adam Warren (3-2), and Andrew McCutchen and Sean Rodriguez homered earlier off Jon Lester. The National League pitcher of the month for June, Lester has allowed 13 runs in 4 1/3 innings in July, including five in three innings Saturday.

“Terrible. Unacceptable. Letdown,” Lester said, describing his outing. “Offense gives me two runs early and then gives me runs again — and then I give it right back. Obviously a letdown, not only for myself but for the team.”

Ben Zobrist had a two-run homer and Anthony Rizzo had four hits for the Cubs, whose division lead is as small as it’s been since May.

The biggest reason why? Starting pitchers that have a majors-best ERA but have allowed 41 earned runs in 41 innings during July.

“It seemed like early on we couldn’t do anything wrong,” Lester said. “And I’ll be the first one to tell you that what we were doing early on was pretty impressive. But just like with how we were playing overall and the pace we were on to win, what was it, 120 games or whatever, we all knew that that was not realistic.

“But at the same time, we’ve got to be better as a staff, we’ve got to be better as a whole and that starts with me tonight.”

Bell slugged a fifth-inning Warren pitch over the seats perched above the 21-foot wall in right for his second pinch-hit in two games since being recalled from Triple-A on Friday.

“What a fantastic sequence,” said the Pirates’ Clint Hurdle, who has more than 1,000 managerial wins. “I got goose bumps; I’ve got them probably a dozen times since I’ve been a coach or a manager.

“Chills went down my spine. … Seemed like the stadium was pretty excited, too.

Chad Kuhl lasted only 2 1/3 innings in his third major-league start, surrendering Zobrist’s 13th home run among seven hits allowed. But Pittsburgh’s bullpen was much better as Arquimedes Caminero (1-2) struck out three in 1 2/3 innings and Jared Hughes earned his third major-league save by pitching three scoreless innings.

At least one run was scored during eight of the first 11 half-innings played on a night in which the city of Pittsburgh celebrated its bicentennial. A 25-minute fireworks display set off just a few hundred yards away at the city’s Point State Park provided a soundtrack of regular rumbling booms throughout the fifth inning in which Bell and Mercer thrilled the Pirates’ third sellout crowd of the season even more.

ALL-STAR COMINGS AND GOINGS

Cubs OF Dexter Fowler backed out of playing in Tuesday’s All-Star game as a precaution as he recovers from a strained right hamstring that has him on the disabled list.

Pirates OF Starling Marte was announced as one of the NL roster replacements along with the Reds’ Jay Bruce for Fowler and the Mets’ Yoenis Céspedes.

ROSTER MOVE

The Cubs recalled INF Munenori Kawasaki from Triple-A and Saturdayand optioned INF Jeimer Candelario to Iowa. Maddon said the move was made to give the team a more experienced bat off the bench.

TRAINER’S ROOM

Cubs: Maddon said Fowler did not experience a setback in a minor-league rehab appearance Friday. He also said he would have had no problem if Fowler had made his return from the disabled list in the All-Star game and not for the Cubs — “as long as he was well enough to play, and then we got him on Friday.”

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