The win was the sixth in the last seven games for the last-place D-backs, who swept the three-game series from the first-place Padres, who have now dropped seven straight.
"The guys are playing good baseball and hopefully we can keep it going," D-backs outfielder Chris Young said. "It's nice to know that you can make a difference. Hopefully the race can get a little closer for [the Padres] because that's what's fun to watch."
Allen was fun to watch Wednesday. A first baseman throughout his career, he began playing some left field for Reno this year as a way of adding to his versatility. The 24-year-old was told following Reno's game Tuesday night that he was headed back to the big leagues where he spent 32 games last year.
Because he had to pack up his apartment and catch an early flight, Allen went without sleep and came straight from the airport to Chase Field around 8 a.m. MST. Then he learned he would be playing the outfield in the big leagues for the first time ever.
The first ball hit in his direction was a drive off the bat of Miguel Tejada that landed in the left-field bleachers to give San Diego a 1-0 lead.
One out and a single later, Chase Headley lifted a deep fly ball toward the corner in left.
"Tejada's was a little farther, I was like that's kind of far," Allen said with a smile. "Headley's had a little more air under his so I figured I'd go for it, run through the fence the first day and see what it feels like."
Allen caught the ball on a dead run and crashed into the fence by the D-backs' bullpen.
"Unbelievable," Arizona starting pitcher Barry Enright said. "I cannot say enough."
Allen, though, was far from done.
After Young's homer off the left-field foul pole tied the game at 1, the Padres managed to push across one in the seventh on a grounder back up the middle by Will Venable.
In order to get that run, the Padres had to pinch-hit for starter Mat Latos, who was dominant in striking out 10 over six innings. San Diego manager Bud Black then turned to his National League-best bullpen, but Luke Gregerson struggled with his command, Tejada made an error and the D-backs managed an infield single to load the bases.
"We were fortunate," Gibson said. "There was a little bit of luck on our side with some of those hits, so we'll take it."
That brought Allen, who was 1-for-2 with a bloop single, to the plate.
"Bases loaded, just get an easy sac fly," Allen said of his thought process stepping in the box. "The first pitch, I got a little overanxious on a slider down and just said back it up and get it up and I did and I put a good swing on it."
Allen launched the hanging slider into the bleachers in right for his first career grand slam and a 5-2 Arizona lead.
"He had a pretty good first day for us didn't he?" Gibson said.
Allen was asked if he envisioned having a game like that.
"Nobody imagines that, it just kind of falls into place and let the chips fall where they may," he said.