DENVER -- As Evan MacLane spoke to reporters after his Major League debut, Cardinals pitching coach Dave Duncan went out of his way to walk by the rookie's locker and clap him on the shoulders.

"Little bit of an unfair assignment," Duncan told the left-hander.

It was a severe understatement. MacLane's first big league batter so far is the only one he's faced, as he served up a leadoff, walk-off home run to Chris Iannetta in the ninth inning on Wednesday night. The Cardinals lost to the Rockies, 8-7, making it back-to-back gut-wrenching walk-off defeats for St. Louis.

Thanks to the Reds' win in New York, the Cardinals fell three games behind first-place Cincinnati. That's their largest deficit since May 19, 2009, when they were three behind the Brewers. They led the division by five games on May 4 of this year.

The Cardinals called up MacLane from Triple-A Memphis on Wednesday afternoon, and he didn't arrive at Coors Field until about the fifth inning. When St. Louis went through four relievers to get from the sixth through the eighth, the ball was handed to MacLane for the ninth. The Cards had already let leads of 5-0 and 7-4 turn into a tie game, and it was MacLane's job to keep the rolling Rockies at bay.

It was too much to ask of him. He hung a 3-2 changeup to Iannetta, and the Colorado catcher crushed it for the game-ender.

"It's tough," he admitted. "But I'm up here and I've got to do my job. I've got to pitch. I've got to get these guys out. That's the reason I'm here. Not the ideal situation, but when is there going to be one? I've got to show up and be ready. I'm ready."

Just as was the case on Tuesday, at least part of what happened in the ninth was set up long before. A missed double play in the fifth cost starter Jaime Garcia dearly, and a three-pitcher, three-run eighth put MacLane and the Cardinals in the unenviable position they faced in the ninth.

With St. Louis leading by five, Garcia seemed to be gaining some momentum as the game went to the bottom of the fifth. Brad Eldred led off with a single to left that third baseman Felipe Lopez couldn't corral, but Clint Barmes hit a ball to shortstop that should have gone for a double play. Instead, Tyler Greene's throw went well wide of Skip Schumaker, and Colorado had men on second and third with no outs.

The next batter was pinch-hitter Ian Stewart, and Stewart singled home two runs. Two more singles cut the lead to 5-3, and the Cards were in a much tighter spot than they had been.

"I just got on the side and it just got away," Greene said.

Still, the Cards kept adding runs. Colby Rasmus singled in the sixth run in the sixth inning, and Matt Holliday's second homer made it 7-4 in the seventh. However, in the bottom of the eighth, the Cardinals had another tough stretch. Eldred led off with a double, and after Brendan Ryan made a tough play to retire Barmes for the first out, manager Tony La Russa summoned Trever Miller. For the second straight night, the lefty didn't retire a batter, walking Stewart.

On came Jason Motte, who allowed a three-run homer on a 3-2 fastball to Dexter Fowler. He got out of the rest of the inning with no further damage, but after throwing 25 pitches, he did not start the ninth.

"Boggs needed to get the leadoff guy," La Russa said. "That was a big miss. And then Trever needed to get the left-hander out. That puts us in a bad spot. But that's the life of the reliever."

With Dennys Reyes not available due to illness, and the Cards holding Ryan Franklin to pitch only in a save situation, the assignment fell to MacLane. His big league debut lasted six pitches.

"Not a good [situation]," La Russa said. "But that's why he's here, to pitch when needed. If we'd scored in the top of the ninth, we could have pitched Franklin, even though it would have been a big push. But Reyes was sick, and that's not his situation. That's why you've got Evan in that spot. Tie game, he's got to pitch on the road."

For the second straight night, not only did the Cardinals take a tough loss, but the defeat obscured some good things earlier in the game. Once again, the St. Louis starter turned in a good game in tough conditions. And once again, the offense put on a show, highlighted by Holliday. The former Rockies star has three homers in two games of this series.

But it has been lost as Colorado has risen to the occasion in the late innings.

"No matter what's going on, guys are constantly trying not to give any at-bats away," Barmes said. "Just because we're down early in a game or whatever, no matter what the deficit is, we're going to start chipping away."