Sandoval's three-run homer with one out in the fifth chased Maholm and was followed back-to-back by a Buster Posey blast that greeted reliever Jose Dominguez in a four-run inning. Dominguez had pitched two innings a day earlier in relief of Hyun-Jin Ryu's shortest Major League start.
To get through the final 4 2/3 innings Saturday, Mattingly used six relievers, including 22 pitches from closer Kenley Jansen while down by five runs. Jansen needed the work, not having pitched since Tuesday with a day off Monday.
But it's clear the Dodgers are already missing the innings usually eaten with excellence by Clayton Kershaw, who would have started one of these two games against the Giants if he wasn't on the disabled list.
Maholm, essentially the sixth starter forced into action by injuries to Kershaw and Josh Beckett, was charged with five runs on seven hits in 4 1/3 innings, rusty in his first start since last year after a Spring Training strange by even this year's Dodgers standards.
Maholm has been on standby, not really a starter, not really a reliever, not knowing when or for how long he would or wouldn't pitch.
"The toughest thing is not having a normal Spring Training as a starter and not knowing how I'll be used next," he said.
"He's been the guy yanked around more than anybody of the starters," said Mattingly. "He showed he can get outs and be OK. I think this is a pretty tough matchup for him."
Maholm couldn't match Giants ace Madison Bumgarner, who contained a Dodgers lineup loaded with right-handed bats that included Yasiel Puig, who returned after being benched for showing up late to Friday's home opener.
Puig went 1-for-4 and injured his left thumb when he failed to get all the landing gear down on a head-first crash onto first base to end an infield single. He jammed his thumb on the bag but taped it and stayed in the game. After the game, he said he was waiting for X-ray results, but the thumb wasn't taped or splinted.
Puig was joined in the lineup by Matt Kemp, playing back-to-back days after coming off the disabled list for ankle surgery.
In fairness to Maholm, Dodgers bats are usually useless against Bumgarner, who is 9-3 with a 2.60 career ERA against the Dodgers. He went 6 1/3 innings and was charged with two runs on eight hits, striking out 10 with a walk. The Dodgers were 2-for-12 with runners in scoring position.
The Giants struck in the second inning. After a tough backhand play by third baseman Juan Uribe robbed Posey of a leadoff hit, San Francisco put the next three batters on base with a walk to Morse and singles by Brandon Belt and Brandon Hicks.
With the bases loaded, second baseman Justin Turner was able to get a forceout on Brandon Crawford's bouncer to the right side, but the ball wasn't hit hard enough for a double play and Morse scored.
The Dodgers tied the game in the bottom of the inning. Scott Van Slyke walked, and when third-base coach Lorenzo Bundy saw that center fielder Angel Pagan couldn't cut off Uribe's double to the alley, he waved home Van Slyke.
Morse was apparently looking for something offspeed when he led off the fourth inning by crushing a changeup to left-center for a 2-1 Giants lead.
Sandoval's three-run shot came on a cutter that "didn't cut," according to Maholm. "When they don't, they usually end up where it did. Maybe a sinker away and I get a double play. You can always look back."
The Giants added to the lead in the sixth off Paco Rodriguez with a one-out walk to Crawford and a two-out double by Pagan.
The Dodgers chased a tiring Bumgarner in the seventh inning and scored on Andre Ethier's two-out, bases-loaded pinch-single, but ran out of a chance for a bigger inning. On Ethier's single, with Hanley Ramirez on deck, Bundy waved home A.J. Ellis trying to score from second base and he was thrown out at the plate by Hunter Pence.
"That's something to talk about internally," Mattingly said when asked about the decision to send Ellis home.
Mattingly, though, asked for a replay review from plate ump and crew chief Joe West on the tag play at the plate, but the out call was confirmed and the inning over.