"I got up there, was in good position and it went off the tip of my glove," Upton said. "I couldn't really tell what happened, I just didn't get the pocket on it."
The long ball came with two outs in the eighth inning when the score was tied, 4-4. Fittingly, Upton grounded out to end the game in the ninth inning to finish 0-for-5 on the night.
He left the field to the sound of boisterous boos as the Padres swept the D-backs at Chase Field for the first time in franchise history.
"To be honest, I don't care anything about what the fans think of me," Upton said. "My teammates, my coaches, they know I bust it every single day. I do everything to help this team.
"The fans can think whatever they want to think. They can call my lazy, I've heard that. They can call me washed up. Whatever they call me, at the end of the day, I'm thankful for every opportunity I get on the baseball field and I try my hardest every day."
Including Wednesday, Upton is 1-for-18 in his past four games.
"I remember what it was like, nobody wants to be booed," D-backs manager Kirk Gibson said. "He's playing his tail off, he's frustrated. It's not like he's not trying. It's not the results he wants, but sometimes the game is a cruel thing."
For one, Grandal initially thought Upton had taken away his home run when he first saw the outfielder leap and make contact with the ball.
"When I saw him jump, I knew it was out but I thought he was going to catch it," Grandal said. "Once I saw that ball drop, it was a relief. It happens. Maybe in another situation, he comes up with it. Today, it went my way, and another day, it goes their way."
Wednesday's loss now has the D-backs reeling. At two games over .500 just five days ago, Arizona now sits three games below the even mark.
Jason Kubel provided the majority of the offense for the D-backs with one swing on a three-run shot off of Padres starter Jason Marquis. Since the beginning of June, Kubel has driven in 35 runs and hit 10 homers. His 58 RBIs on the season matches his total from 2011.
D-backs starter Ian Kennedy again didn't have the stuff that made him a 21-game winner last season but pitched well enough to last seven innings. The club's ace allowed nine hits and four runs while striking out five.
"I felt like tonight was a battle to get through," Kennedy said. "They are tough outs, not one of them is easy. They have a pretty good lineup, it's a bunch of scrappy guys that put together good at-bats. I think their win-loss numbers are deceiving."
With the game tied, 3-3, and two outs in the top half of the seventh, Alexi Amarista stroked an RBI base hit to drive home Chris Denorfia, who singled one batter earlier and stole second base. The D-backs responded in their half of the seventh to tie the game, 4-4, when Willie Bloomquist singled in Gerardo Parra, who also stole second base after a hit.
David Hernandez took over in the eight and quickly retired the first two batters he faced before hitting Logan Forsythe and serving up the game-changing long ball to Grandal on an 0-2 fastball down the middle of the plate.
"He made a mistake there," Gibson said. "He was trying to elevate it and Grandal obviously is swinging a hot bat. That hurt, that stuns you when you climb back in a game and then they come right back."
For the third straight night, a D-backs pitcher committed an error when Bryan Shaw fielded a bunt in the ninth inning and threw it into left field trying to snag a runner at third base. The Padres went on to score two runs in the frame before closing out the D-backs minutes later.
"They just had our number," Gibson said. "They outplayed us. It's no discredit to our team but they just outplayed us."