PHOENIX -- Ryan Vogelsong shook his head and offered a three-word answer: "I don't know."

In a 10-2 Giants loss to the Arizona Diamondbacks, Vogelsong struck out five of the first nine hitters he faced and then fell apart. He was knocked out in the fourth inning Sunday, continuing a late-season slump for a pitcher who led the National League in ERA when he took the mound Aug. 13.

In seven starts since, Vogelsong has a 10.31 ERA.

"I don't know," he repeated, when asked what has gone wrong. "I can't say that I missed location today. Only one pitch missed location."

That was Justin Upton's double in the third inning that nearly cleared the wall in right-center -- 413 feet away from the plate -- and gave the Diamondbacks a 2-1 lead. The crushing blow came an inning later, when opposing pitcher Patrick Corbin roped a bases-loaded triple inside the first-base bag.

"Heater up and in, and it wasn't even a strike," Vogelsong said in wonderment

Manager Bruce Bochy has dealt with slumping pitchers before, most notably in the first half this season, when Tim Lincecum went through the worst slump of his career. As he did with Lincecum, Bochy continues to stand firmly behind Vogelsong.

"He's one of our guys, and I have all the confidence in the world in him," Bochy said. "Every time he goes out there, I feel good about what's going to happen. He'll come out of this."

Vogelsong took the only losses on a 4-2 trip, giving

up 10 earned runs in 81/3 innings against the Colorado Rockies and Diamondbacks. He was charged with six earned runs Sunday on five hits and four walks. One of the walks was intentional and loaded the bases for Corbin, who had only two hits and no RBIs in his first 35 career plate appearances.

The rookie hit a sharp grounder down the first-base line and put the Diamondbacks ahead 5-2. Vogelsong's day was done a batter later, after he walked Adam Eaton on four pitches. It was another stunning turn for a pitcher who struck out three in the second inning and had gotten Corbin looking in the third.

"Again, he had really good stuff," Bochy said. "The good news is his shin (which was hit by a batted ball Monday) is fine, and his strength is good. He's just going through a tough period. I'm encouraged with his stuff."

The strong stuff, coupled with another day of rough results, left Vogelsong wondering where it's all going wrong.

"It's getting perplexing," he said. "My stuff is there. It just doesn't make much sense to me right now. I feel healthy, I feel strong -- the results aren't there."

Corbin pitched like the Vogelsong of the first half, throwing just 93 pitches while getting through the eighth inning for the first time in his career. The Giants managed just a Buster Posey RBI single and a Brandon Belt RBI double.

Vogelsong is used to being the one who walks off the mound with a lead and a quality start in his back pocket. He expects to be that guy again.

"I don't see how I can keep throwing the ball the way I am and get the results I am," he said. "I've just got to keep fighting. It'll turn around -- it has to."

Center fielder Angel Pagan was pulled in the bottom of the sixth with back tightness, but Bochy expects Pagan to be in the starting lineup Monday when the Giants start a four-game series with Colorado. Bochy said the back tightness is something Pagan has dealt with occasionally this season.

Left fielder Gregor Blanco sat out Sunday's game with a sore left shoulder. Blanco said the shoulder first began hurting Saturday night, but he wasn't sure what the cause of the pain was.

"I've been asking myself that since (Saturday)," he said. "This is the first time in my career I've hurt my shoulder. I don't know what happened."

Blanco was checked out by team doctors and is hopeful he can play Monday. Since Melky Cabrera was suspended, Blanco has started 19 of 29 games in left field.

Vogelsong's short day allowed Bochy to use several relievers who had found trouble getting into games of late. Clay Hensley made his first appearance since coming off the disabled list and gave up a homer to Upton in one inning of work. Brad Penny took the mound for the first time since Sept. 5 and pitched a scoreless inning. In his first appearance since Sept. 4, Shane Loux gave up a run-scoring double.

In all, Bochy used 21 players, including eight pitchers.

"You hate to have a game like this, but the silver lining is that a lot of guys got work," Bochy said. "I'm sure they were rusty."

For more on the Giants, see Alex Pavlovic's Giants Extra blog at blogs.mercurynews.com/Giants. Follow him on Twitter at twitter.com/AlexPavlovic.