PITTSBURGH -- Pittsburgh Pirates first baseman Josh Bell's introduction to Major League Baseball has been a dream.

After singling against Chicago Cubs ace Jake Arrieta in his first Major League at bat Friday, the 23-year-old rookie stepped to the plate for his second appearance, with the bases loaded and one out, a day later. Facing right-hander Adam Warren, Bell cracked a foul ball to left field, building a buzz throughout PNC Park, before launching a bomb that carried over the right-field wall and bounced into the Allegheny River.

Pittsburgh's third-ranked prospect, according to Baseball America, gave the Pirates an 11-5 lead that would balloon to a 12-6 win against the Cubs on Saturday. He stepped out of the dugout for a curtain call and soaked in the raucous applause from the Pittsburgh crowd.

"Those two, three minutes felt like an hour and a half," Bell said. "It's just everything that a kid wishes for as a baseball player, just growing up playing in the backyard. Bases loaded, first big league home run. I can't ask for much more."

After promoting Bell from Class Triple-A Indianapolis on Friday, Pirates manager Clint Hurdle said he'd use him as a bat off the bench during the weekend series. That decision has paid dividends.

"What a fantastic sequel," Hurdle said. "I got goosebumps. I've got them probably a dozen times since I've been a coach or a manager. I think (David )Freese was standing next to me. I told him I had goosebumps and chills ran down my spine. It seemed like the stadium was pretty excited too."

Shortstop Jordy Mercer followed Bell's grand slam with another homer, making it 12-5 through five innings.

Warren took the loss to fall to 3-2 after allowing seven earned runs in two innings.

"I'm not going to say I was 100 percent," Warren said. "But we're short on arms so somebody has to go out there and pitch. I felt like at the beginning I was at least able to go out there and throw."

The Pirates (46-42) pulled to within 6 1/2 games of the National League Central-leading Cubs (52-35) with their ninth win in 10 games. Chicago's recent struggles continued with a fifth consecutive loss, its ninth in 10 games.

Chicago got a run back in the top of the sixth when Ben Zobrist grounded into a double play, allowing Kris Bryant to score from third.

Pirates right-handed pitcher Chad Kuhl was removed after 2 1/3 innings in his third major league start after allowing the Cubs to tie the game 3-3 in the top of the third inning.

Anthony Rizzo led off the Cubs' third with a triple to left-center field before Zobrist grounded to first base. Willson Contreras drove in Rizzo with an infield single, chasing Kuhl from the game.

Right-hander Juan Nicasio relieved Kuhl and struck out Addison Russell before allowing an infield hit to Javier Baez that scored Contreras from second base, giving Chicago a 4-3 lead.

Cubs left-hander Jon Lester didn't fare any better. After allowing three runs in the second, he surrendered a lead-off home run to Andrew McCutchen, his 13th of the season, to left field in the third.

Lester responded by striking out Jung Ho Kang and forcing Starling Marte into a ground out, but allowed a second home run to left when Sean Rodriguez hammered his curveball 412 feet to give the Pirates a 5-4 lead after three innings.

After another poor outing, which Lester referred to as "terrible," he didn't allow excuses.

"I don't know why if you pitch poorly, it has to be a physical reason," Lester said. "No, we all go through times where, just like hitters, you go through slumps. And you've got to find ways to contribute. Obviously, the last two starts were null and void for me. 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. It'll be better."

The pendulum continued to swing in the top of the fourth inning.

Nicasio surrendered back-to-back doubles to start the fourth, with the second coming off Rizzo's bat, to tie the game 5-5. After walking Contreras to put runners on first and third with one out, Nicasio was pulled in favor of right-handed pitcher Arquimedes Caminero, who retired his first two batters faced to preserve the 5-5 tie.

McCutchen gave the Pirates their third lead of the game with a two-out double that drove in two runs in the fourth inning, pushing Pittsburgh to a 7-5 advantage.

NOTES: The Cubs moved RF Ben Zobrist, who played second base Friday, to the cleanup spot behind 1B Anthony Rizzo. Zobrist hit lead-off in Chicago's 8-4 loss Friday. ... Tommy La Stella replaced Zobrist at second base. La Stella failed to get on base when he pinch-hit on Friday. ... Cubs C Willson Contreras moved back behind the plate on Saturday after starting in left field Friday. Kris Bryant transitioned from right to left field. ... Pirates LF Starling Marte was named to the National League's All-Star team as an injury replacement Saturday. Marte led Pittsburgh with a .316 batting average entering Saturday. ... Pittsburgh 3B Jung Ho Kang returned to the lineup and hit cleanup after taking a day off Friday. David Freese, who played third in place of Kang on Friday, moved across the diamond to first base. ... Pirates C Eric Fryer replaced Erik Kratz behind the plate to catch RHP Chad Kuhl in his third major league start. ... Pirates C Francisco Cervelli took part in batting practice on Saturday. He has not played since breaking the hamate bone in his left hand against the St. Louis Cardinals on June 10.