CHICAGO -- Their bullpen fried after their previous two starters failed to last three innings, the Arizona Diamondbacks needed a long, effective outing from Ian Kennedy -- who hadn't won a major league game in 2 1/2 years.

Kennedy delivered the improbable Thursday -- and reminded everybody why he once was one of the New York Yankees' top prospects.

Staked to a 12-run lead by an attack that featured Adam LaRoche's two homers and five RBIs, Kennedy worked eight strong innings in Arizona's 13-5 victory over the skidding Chicago Cubs.

"Obviously, nobody expected this," LaRoche said. "It's amazing. We all hope he keeps it up."

Kelly Johnson and Chris Snyder also connected for Arizona, which leads the majors with 33 homers. Johnson, who has an NL-high nine homers and has gone deep six times in the last seven games, added three singles.

Still, it was Kennedy who really came through with what the Diamondbacks needed after Tuesday's 12-1 loss at Colorado was followed by Wednesday's wild 12-11 victory.

"There wasn't a guy in the bullpen who didn't need a day off," Arizona manager A.J. Hinch said. "(Kennedy) had the fastball down and away whenever he wanted. As the game went on, it looked like he could get deep."

Kennedy (1-1) allowed only one run in the first seven innings before giving up Kosuke Fukudome's grand slam in the eighth. He ended up allowing six hits, one walk and four earned runs while striking out six.

Kennedy hadn't won since he beat Tampa Bay in his big league debut on Sept. 1, 2007. The right-hander's next two seasons in New York were filled with ineffectiveness and injuries -- including an aneurysm near his right shoulder that required surgery last May.

Acquired on Dec. 9 in the three-team trade that sent Curtis Granderson from Detroit to the Yankees, Kennedy allowed nine runs in 9 1/3 innings in his first two Arizona starts. In his last three, however, he gave up only six earned runs in 21 innings. He has gone eight innings in each of his last two outings.

"And both came when we needed it," Johnson said. "We'll take that every time."

They'll take a 14-hit, four-homer, 13-run attack every time, too.

LaRoche hit a three-run shot after Ted Lilly (1-1) walked the first two batters in a five-run fourth inning. The next inning, LaRoche lined a homer through a 27 mph wind. And in the seventh, he doubled home the first of seven runs. It was LaRoche's 13th career multiple-homer game, and the five RBIs tied a career high.

"We're showing what we can do throughout the whole lineup," LaRoche said. "We're seeing the ball good, getting hits, and then you've got two or three guys behind you to pick you up."

While the Diamondbacks (11-11) got back to .500 for the first time since April 16, the Cubs lost their third straight to fall a season-high 5 1/2 games behind NL Central-leading St. Louis.

"We've got to have a little urgency," manager Lou Piniella said.

Asked if he was criticizing his team's effort, he said: "No, I'm saying in terms of wins and losses. They're trying, believe me. We're only a few games under .500. We just need to stabilize this thing and win some games and get above .500. That's what our job is."

Making his second start after recovering from offseason shoulder surgery, Lilly allowed six runs on seven hits in five innings. Lilly, who became the first Chicago starter since April 15 to give up more than three earned runs in a game, had pitched six shutout innings in his first outing.

He blamed the loss on lack of command, not on lingering health issues.

"I started off the fourth inning out of the strike zone," he said. "At some point, you're going to have to make pitches to get guys out, and I wasn't able to do that."

Game notes
Of the game's five homers, only Fukudome's first career grand slam appeared to have been helped significantly by the strong, right-to-left cross-wind. ... The Diamondbacks are 53-36 against the Cubs since their inception in 1998, including 24-21 at Wrigley Field. ... Arizona is 8-0 when homering at least twice, 0-6 when failing to go deep. ... Diamondbacks RHP Daniel Stange made his major league debut, pitching a perfect ninth. ... Cubs SS Ryan Theriot, who singled home the team's first run, is batting .488 during a nine-game hitting streak that includes eight multi-hit games. ... Cubs No. 3 batter Derrek Lee has five hits in his last 49 at-bats. He is hitting .193, and cleanup man Aramis Ramirez is at .159.