Seattle moved one game over .500 for the first time since it was 3-2 on April 7.
"I feel like the chemistry on the team is very good," Hernandez said through an interpreter. "All the guys are working as a unit. The pitching is good, the offense is good. We believe in
all the guys. If one guy doesn't do it the next guy picks him up."
Kenji Johjima's sixth-inning run-scoring single broke a 2-2 tie and lifted the Mariners to their fourth straight win and ninth in 11 games. Seattle then put the game out of reach with three runs in the eighth and four more in the ninth against the Arizona bullpen.
Shawn Green doubled and had two RBI for the Diamondbacks, who have lost 19 of their last 22. The Diamondbacks, who led the NL West by 2½ games on June 6, now trail the first-place Padres by 4½ games.
"Last year we went through some tough stretches and this is pretty much the same type of year," Green said. "It's not a whole lot different from that. You've just got to try and get on a roll again and get the energy changed to a feeling of invulnerability rather than a feeling of failure."
With the score 2-2 in the sixth, Jose Lopez doubled to the warning track in left off Edgar Gonzalez (0-2) and went to third on a grounder by Raul Ibanez. After Gonzalez struck out Richie Sexson on three straight pitches, Johjima lined a single to right, scoring Lopez for a 3-2 lead.
The Mariners put the game out of reach with three runs in the eighth. Adrian Beltre and Lopez hit consecutive triples and Ibanez singled to make it 5-2. After a walk to Johjima, Reed singled home Ibanez for a 6-2 lead.
Lopez finished 3-for-5 with a double, a triple and three runs scored.
"Everybody's relaxed," Lopez said. "We're playing happy."
Green cut it to 6-3 with a sacrifice fly in the eighth, but Sexson hit a three-run double and scored on Johjima's single in the ninth to make it 10-3.
"We're just playing bad," said Arizona manager Bob Melvin. "It's not like we're losing 2-1 every night or even 10-9. We're getting beat and that's a difficult pill to swallow. Early in the season we were in every game. Right now we're not playing well enough to even be in games."
Hernandez (8-7) held the Diamondbacks to the two runs and seven hits while walking one and striking out four for his fifth win in six starts.
"Early on he was searching for confidence," Hargrove said of the 20-year-old right-hander. "Now he has that confidence and he's making his pitches. You can see it on his face."
Seattle had taken a 1-0 lead in the third when Yuniesky Betancourt singled and scored on Beltre's bloop double to right. The Diamondbacks answered in the fourth, tying the game when Luis Gonzalez was hit by a pitch and scored on Green's double.
Green moved to third on Johnny Estrada's fly ball to center and scored on Orlando Hudson's grounder to third for a 2-1 Arizona lead. Beltre scooped the ball halfway down the line and looked home briefly before throwing to first for the sure out.
The Diamondbacks' lead lasted two pitches into the fifth when Reed homered into the pool area in right-center field, clearing the fence by inches.
"I'm still trying to get there but the last few at-bats have been comforting," said Reed, who earlier this season went through an 0-for-21 slump.
Edgar Gonzalez allowed three runs and eight hits over seven innings in his second start of the season. He walked one and struck out four.
Beltre has hit safely in 11 straight, 19 of his last 20 and 28 of his last 30 games. ... The Mariners lead the AL with 27 triples. ... Arizona third baseman Chad Tracy went 2-for-3 for his ninth multi-hit game in June and 24th of the season. ... Hernandez has allowed two runs or less in three of his last five starts. ... Diamondbacks first baseman Conor Jackson walked in the eighth and leads ML rookies with 34. He also is in the top 10 in every category except triples and stolen bases. ... Five of Reed's six homers have come on the road. ... Edgar Gonzalez singled in the fourth for his first major-league hit since Aug. 13, 2004. He has not won a start since his major-league debut June 1, 2003, at San Diego. ... Ichiro Suzuki went 2-for-4 and has hit safely in five straight since his 20-game hitting streak was snapped June 22 against the L.A. Dodgers.