CINCINNATI -- The Diamondbacks had a sweep on their minds, they might have even had it penciled in, but they couldn't put it in ink, because Jose Valentin hit a pinch-hit, walk-off home run to lift the Reds to a 5-4 decision in front of 27,694 at Great American Ballpark.

Taking a 4-1 lead into the eighth, the D-Backs looked like they were going to reward rookie Enrique Gonzalez with a win in his Major League debut. The rookie hurled six excellent innings, allowing just three hits and one run while striking out four.

But, the effort was squandered -- as the usually rock-steady bullpen let it slip away. Brandon Lyon allowed a two-run shot in the eighth to Scott Hatteberg to cut the lead to 4-3. Then, in the ninth with closer Jose Valverde looking to shut the door for his 15th save, the D-Backs had to get out the eraser if they thought they were about to chalk up their 30th win of the season, and change it to their 20th loss.

And it happened quickly -- with one out Austin Kearns dribbled an infield single, and Valentin followed by just squeaking one down the right field line for the heroics.

"To his credit, it was a splitter that Valentin hit, and every time we've faced him he's swung at the first pitch looking fastball, and looking pull," manager Bob Melvin said.

"I don't think he got all of it, but he certainly hit it in the right spot, and I don't second-guess the pitch," he added about Valverde's offering.

Melvin also knows that the bullpen can not always be perfect.

"It happens. We're not going to go the whole year with not giving runs down there," Melvin said.

"Granted, the way we've been down there, you can certainly expect [good] things."

Arizona jumped on starter Bronson Arroyo with singles by Craig Counsell and Chad Tracy, before Luis Gonzalez hit his 512th career double to put him in 35th on the all-time list as the D-Backs grabbed the quick 2-0 lead. Conor Jackson added a single that scored Gonzalez to make it 3-0.

Ryan Freel took Enrique Gonzalez's fourth Major League pitch and deposited a leadoff home run into the left field seats to cut it to 3-1.

"In the first inning I was feeling a little nervous. You don't face this many [fans] in the Minor Leagues," Gonzalez acknowledged.

And as he watched Freel's smash sail to the seats, the youngster admitted his thoughts on the mound: "He got me. Welcome to the Big Show." It was the last noise that the Reds bats would make off of Gonzalez though, as he settled into a groove over the next six innings of work in which he threw 86 pitches, with 54 coming across as strikes.

"I came in here and pitched my game, and I became more confident," Gonzalez said.

"I thought he was great," Melvin began.

"First few pitches he might have had a little bit of nerves, yanked some balls off the plate and gave up a home run to the first guy, but after that he was great, just phenomenal.

"You can see why everybody is so high on him. He popped it up there at 95 [mph] one time, he has good sink and actually has more pitches than I realized -- slider, curve and change -- and he used them all. He really got stronger as the game went on."

Now, Gonzalez will just have to wait and see if he has at least temporarily earned a spot in the rotation as he would be due up next to pitch over the weekend at Atlanta.

Enrique Gonzalez not only got an offensive boost from his left-fielding namesake, but Luis Gonzalez also added a defensive gem in the sixth when he tracked down Freel's sinking liner with a superb sliding effort.

The defense was sound all day. In the fifth, Tracy dove to his left to stab catcher David Ross' grounder, and quickly came to his feet to make the throw. Center fielder Jeff DaVanon tracked down a long drive by Ken Griffey Jr. to tack on another nice defensive play in the sixth and end the Reds threat.

Arizona added a run in the top of the sixth when Orlando Hudson's bases-loaded groundout brought Jackson across to make it 4-1.

Jorge Julio pitched a scoreless seventh, before Lyon's trouble in the eighth. After he gave up the Hatteberg home run, he then put runners on the corners with one out, before striking out Rich Aurilia and getting Adam Dunn to line out to left to escape the self-inflicted jam.

Before the Reds started their rally, manager Jerry Narron gave the team a shot of adrenaline in the seventh inning, and got the Cincinnati faithful back into the game by getting ejected by home-plate umpire Larry Vanover for arguing an Adam Dunn strikeout.