Oakland, Calif. ? Jason Vargas dueled with Jon Lester through four impressive innings untouched. Then, everything imploded in a matter of pitches.

And it was over.

Vargas came off the disabled list to make his first start since undergoing an appendectomy July 9, getting knocked out in that decisive fifth inning of an 8-3 loss to the Oakland Athletics on Saturday.

After the first 12 A’s batters were retired in order, the next 12 produced eight hits and eight runs.

“I left some balls up, we didn’t play defense and that’s what happens — unraveled,” said Vargas (8-5). “As a whole we weren’t very good in the fifth inning. That’s what it comes down to. I wanted to get through the fifth a lot better than we did, but we didn’t.”

Lester worked into the seventh inning to win his A’s debut and Jonny Gomes produced two timely hits in the fifth, two days after they were traded by the last-place, reigning World Series champion Boston Red Sox.

Pitching on seven days’ rest after being scratched Wednesday ahead of the trade deadline, Lester improved to 8-3 with a 2.20 ERA in 12 career starts against the Royals. He hasn’t lost in nine starts overall since June 7 at Detroit.

“Jonny Lester is one of the best pitchers in the American League,” Royals manager Ned Yost said. “He takes you deep in games every time he pitches. He goes out there and gives you a chance to win it every time he pitches.”

Still, Kansas City has a chance at a series win in today’s finale after a 1-0 victory Friday. Yet the Royals must face their third double-digit winner in three days.

Vargas’ return meant more than the result.

“It was tough not to have him in the rotation for a couple starts,” left fielder Alex Gordon said. “To see him come back and not rusty at all was good to see. He just had that one big inning. But we really didn’t help him that much defensively either. It was just a tough inning.”

The 6-foot-4 Lester (11-7) instantly became an imposing presence on the mound in the pitcher-friendly Coliseum. Fans in the crowd of 30,097 cheered as he went through his warmup routine in left field, then Lester tipped his cap to a roaring ovation when he left after throwing 6 2-3 innings and 104 pitches.

He would have rather finished the inning, but appreciated the gesture.

“To walk off to the ovation was great,” Lester said. “It kind of makes you feel welcome.”

Back with the AL West-leading A’s, Gomes delivered in an eight-run fifth inning. He singled and scored off Vargas, then hit a hit a two-run single off Aaron Crow.

Replay review

In the fourth, the umpires went to replay to determine whether A’s catcher Derek Norris improperly blocked the plate when he tagged a sliding Gordon. The out call was confirmed, with newly acquired center fielder Sam Fuld’s throw resulting in a sensational double play.

“It was a close play. I thought I had made a good slide,” Gordon said. “Obviously he tagged me a little bit before I got to the plate. Sam Fuld made a great throw.”

Trainer’s room

Royals first baseman Eric Hosmer was put on the 15-day disabled with a stress fracture of the third finger on his right hand. He was injured Thursday and is expected to miss 3 to 6 weeks.