ST. LOUIS -- Vince Velasquez's efficiency struggles continued on Monday night.

For the second straight start, Velasquez was unable to record an out in the fifth inning. Despite carrying a no-hitter into the fourth on Monday, walks and home runs caught up with him in a 6-0 loss to the Cardinals in the opener of a three-game series at Busch Stadium.

Velasquez walked five (one intentional), with his fourth free pass coming in front of a Yadier Molina homer -- the Cardinals’ first hit of the game -- to make it 2-0.

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The right-hander then gave up home runs to Matt Carpenter and Paul DeJong in the fifth before leaving without retiring a batter in the frame. Three of the four hits Velasquez served up left the park, and all three home runs traveled more than 400 feet, with DeJong’s measuring 419, according to Statcast.

“I think Vince had good stuff,” Phillies manager Gabe Kapler said. “Unfortunately, he wasn’t able to put hitters away again, and I think the thing that stands out to me is the walks. That was the biggest difference between the two starting pitchers. I don’t think Vince got beat with the home runs, I think Vince got beat with the walks.”

Velasquez needed 98 pitches to go four-plus innings. It was his fourth straight start of at least 97 pitches, but he has averaged fewer than five innings per start during that span.

The problem, against the Cardinals at least, may have been predictability. Velasquez admitted to going to his fastball too much.

“I should do a better job of trusting J.T. [Realmuto] back there,” Velasquez said of his catcher. “He does a great job of studying the guys, knowing what works, and it seems like my secondary pitches were working a lot better than I thought, forcing some ground balls."

Velasquez shook off Realmuto, his catcher, in two key situations. Realmuto wanted either a two-seam fastball or breaking ball against Molina and a breaking ball against DeJong. Velasquez shook him off both times, opting instead for his four-seam fastball in both cases -- and both went over the fence.

Realmuto said it was the first time this season where he and Velasquez had trouble getting on the same page.

“It’s hard to get into a rhythm when you’re up there thinking, trying to think throughout the ballgame,” Realmuto said. “That’s my job to do the thinking, and he’s best when he just gets up there and focuses on execution. That’s all of our pitchers. Guys work best when they’re focused on executing the pitch and they let [Andrew Knapp] and I do the game-calling. And we do our preparation, we know what guys’ strengths and weaknesses are, and where to go and when to go to it.”

The high pitch count played directly into the Cardinals' hands.

"We like to get pitch counts up,” Cardinals manager Mike Shildt said. “There’s no question about that. We have a feeling that if we’re taking good swings on good pitches, then we’re going to have good at-bats and we’re going to have good results -- and we’re going to have a chance to do damage.”

The short outings have exposed a thin bullpen that is waiting for several key arms, including David Robertson and Victor Arano, to return from the injured list. Three different relievers covered the final four innings after Velasquez’s exit, and the only run allowed was by Edgar Garcia, who made his Major League debut.

“I thought his slider was good,” Kapler said of Garcia. “His fastball command needs to be a little bit better in order for him to establish himself as a Major League weapon for us.”

Cardinals starter Miles Mikolas silenced a Phillies' lineup that had averaged more than six runs over its last four games, with Philadelphia winning three of those contests.

Mikolas gave up three hits, all singles, and faced just one over the minimum in seven innings thanks to a pair of double plays. He struck out five and walked none on 98 pitches as the Cardinals snapped a season-high four-game losing streak.

It was just the second time the Phillies were shut out this season.

“He was just doing a good job of mixing,” Realmuto said of Mikolas. “He was able to mix his fastball, slider, curveball -- all three pitches -- [and] threw strikes with all three of them when he needed to. Just keeping us off balance, getting ahead, and once he got a lead, he really started attacking.”