Chavez was on the hook for four runs and eight hits.
"They made him work pretty hard," A's manager Bob Melvin said. "He was just missing with early-count stuff to where he was getting behind, and they were fouling off a bunch of pitches, made him work, took the ball the other way. Sometimes you have to give the hitters credit, and they had a good approach tonight."
"It boils down to all those foul balls and them putting great at-bats together," said Chavez, who is 3-5 with a 5.04 ERA over his last nine starts. "You can't take that away from them."
"It's nothing I'm looking into as, 'Oh, here we go.' They just put together quality at-bats and they didn't give in when I wanted to expand the zone. That's just a good job of staying within the plan they wanted to do tonight."
For a second straight day, A's relievers were asked to provide at least five innings. By the time the eighth was over, their staff had combined to throw 174 pitches, as the Giants smothered them with 17 hits.
"All night they did it to every one of us that went out there," Chavez said. "They didn't give in, they didn't chase down, they didn't expand the zone. For me, a lot of run, a lot of movement that was on and off, and when they came back over the plate, they went with it, and you got to tip your cap to good hitting, but at the same time you got to make adjustments and I didn't do that tonight."
• Right-hander Angel Castro, called up Friday, allowed two hits and struck out one in a scoreless inning. He took the roster spot of right-hander Arnold Leon, who was sent back to Triple-A Nashville after throwing 3 1/3 innings Thursday.
Jane Lee is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.