OAKLAND, Calif. -- Jorge Soler had missed 51 games for the Chicago Cubs with a strained left hamstring and hadn't had a major league at-bat since June 6 until he came to the plate Friday night in the first inning with two on and two outs against the Oakland A's.

He quickly made up for lost time.

Soler lined the third pitch he saw from rookie left-hander Dillon Overton over the left field fence for a three-run homer, and the Cubs rolled to a 7-2 victory at the Oakland Coliseum.

"Sometimes a guy just needs to show up and then he gets the adrenaline going on, and that was a big hit," Cubs manager Joe Maddon said of Soler, who was activated from the disabled list Friday. "I didn't think it was high enough to go out. It was out so quickly."

The home run was Soler's sixth of the season and first since May 27.

"I was really happy that they called me up to be here today," said Soler, who started at designated hitter in the interleague game. "I was coming ready, 100 percent."

Dexter Fowler led off the game with his ninth home run of the season for the Cubs, who won their fifth straight and improved to a major league best 67-41. Soler's blast made it 4-0, and that was more than enough offensive support for left-hander Jon Lester.

Lester (12-4) allowed two runs on six hits, struck out eight and walked none. He faced the A's for the first time since leaving them as a free agent and signing with the Cubs after the 2014 season. The A's acquired Lester on July 31, 2014, from Boston with outfielder Jonny Gomes in exchange for outfielder Yoenis Cespedes.

"Any time you've been on the home side and come back as a visitor it's different," Lester said. "I enjoyed pitching here. I wish it turned out a little different but I was with a good team and great fans. It's unbelievable to come here as a home player as opposed to a visitor."

Lester went 6-4 with a 2.35 ERA in 11 regular-season starts for the A's. He started a wild-card playoff game against Kansas City and gave up six runs on eight hits over 7 1/3 innings. He got a no-decision in the A's 9-8, 12-inning loss to the Royals.

Rookie outfielder Brett Eibner was recalled from Triple-A Nashville on Friday and hit a solo home run in his A's debut. Eibner, who started in right field, was acquired from Kansas City on July 30 in exchange for outfielder Billy Burns. The home run was Eibner's fourth of the season.

Overton (1-3) had another rough outing. He gave up seven runs on nine hits, including two home runs, over three innings as his ERA rose from 9.33 to 10.97 in his first five major league starts.

Overton has allowed 11 home runs. No major league pitcher since at least 1913 has allowed more than nine home runs during his first five career games.

"Believe it or not, I was actually hitting my spots better tonight," Overton said. "They were just hitting everything I threw. You're going to have days like that.

"Overall I thought everything was working really well. I might have left a few cutters over the plate just a little too much, but other than that I thought my changeup was working pretty good. I got a lot of swing and misses on that."

Soler swung and certainly didn't miss Overton's 1-1 fastball.

"I was just looking for a fastball," Soler said. "He threw me a fastball and I put a pretty good swing on it."

Lester blanked the A's for the first five innings on one hit, Khris Davis' infield single, but Eibner hit Lester's first pitch in the eighth over the left-field fence, cutting the Cubs' lead to 7-1. The A's scored again in the inning when rookie Bruce Maxwell doubled and Marcus Semien brought him home with a single.

The Cubs took a 4-0 lead in the first inning. Fowler hit his fifth leadoff home run of the season and 19th of his career. Fowler launched Overton's 3-2 fastball over the left-center field wall for his ninth home run of the season.

Anthony Rizzo blooped a one-out single to center, Addison Russell singled with two outs and Soler homered.

"I think when the ball's over the middle of the plate to a good hitting team, they forced that ball over the plate," A's manager Bob Melvin said. "They had some good swings with guys on base when it mattered, but we've seen (Overton) pitch better than this."

The Cubs extended their lead to 7-0 in the third. Ben Zobrist led off with a double and Overton hit Russell with a pitch. With one out, Jason Heyward had an RBI single and Javier Baez followed with a two-run double.

NOTES: A's first baseman Danny Valencia was ejected by home plate umpire Mike Eastbrook in the bottom of the sixth inning after taking a called third strike for the second time in the game and arguing the decision. ... The Cubs designated RHP Joe Nathan for assignment, opening a spot of their 25-man roster for OF Jorge Soler (left hamstring strain), who came off the disabled list. ... A's 2B Jed Lowrie (bunion, left big toe) was placed on the 15-day disabled list and will decide whether to have season-ending surgery after consulting with doctors within and outside of the A's organization. ... Oakland RHP Jesse Hahn (strained right shoulder) was placed on the 15-day disabled list Friday. ... RHP Andrew Triggs was recalled Friday from Triple-A Nashville. ... A's strength and conditioning coach Mike Henriques was suspended for reportedly installing a hidden camera July 24 in the team's weight room at the Oakland Coliseum to monitor injured players working out while the team was on a road trip. The A's hired in independent law firm to investigate the matter. A's director of corporate communications Catherine Aker said the law firm "provided the findings to us" and they'll be delivered to the team Monday.