Philadelphia added a run in the seventh when left fielder David Peralta dropped Ben Revere's fly to the warning track, and Montero was given an error for dropping shortstop Nick Ahmed's throw and allowing Wil Nieves to score.
"I thought, What should I do?'" he said. "They make it very hard."
Howard hit a two-run homer in the first, his 16th of the season but second since June 19. He is 3 for 8 since his benching.
"These are a couple of games he can build on and continue to contribute and improve and help the team out," Sandberg said.
Roberto Hernandez (5-8) allowed two runs and four hits in seven innings. He had been 0-5 in seven home starts since beating Washington 1-0 on May 4.
"He got groundballs when he needed them, and he threw enough pitches around the zone and got some hitters to swing the bat," Sandberg said.
Jonathan Papelbon pitched a two-hit ninth for his 25th save in 28 chances.
Nuno, making his fourth start since he was acquired from the New York Yankees, gave up four runs — two earned — four hits and three walks in 6 2-3 innings.
The lefty said the defensive miscues are part of the game.
"No, I'm not frustrated," he said. "I had nice rhythm out there and all of my pitches were working."
Nick Ahmed doubled in a run for Arizona in the second, and Paul Goldschmidt hit a sacrifice fly in the sixth.
NOTES: Arizona's Aaron Hill didn't play after being hit on the right hand on Saturday night. ... Arizona first-base coach Dave McKay wasn't with the team in order to attend the Hall of Fame induction ceremonies for Tony La Russa. McKay served on La Russa's staff for 27 straight seasons. ... RHP Chase Anderson (6-4, 3.58) starts for Arizona on Monday in the opener of a three-game series at Cincinnati, which starts RHP Homer Bailey (8-5, 4.22). ... Philadelphia starts RHP A.J. Burnett (6-9, 3.86) in the opener of a three-game series at the New York Mets, who start RHP Bartolo Colon (9-8, 4.03).