ST. LOUIS -- With the Cardinals' recent surge to the top of the National League Central and breaking away from .500, almost every aspect of their game appears cleaner and crisper.

Except for one.

The fifth rotation spot’s struggles were on full display in the Cardinals’ 8-2 loss to the Astros on Saturday night at Busch Stadium, when Daniel Ponce de Leon made it through just 2 1/3 innings in his third start since taking over that rotation spot.

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Ponce de Leon hasn’t made it through the fourth inning in all three of his starts since replacing Michael Wacha, who has been moved to the bullpen on two separate occasions this season. Wacha has relieved Ponce de Leon in his last two starts.

In the top of the third inning Saturday, Ponce de Leon walked three straight batters and exited with the bases loaded. On the sixth pitch of Wacha’s outing, Carlos Correa took a 95-mph fastball over the wall for his first career grand slam and gave the Astros a six-run lead.

“I sucked,” Ponce de Leon said. “Walked four guys in three innings, didn’t give our team a chance.”

Ponce de Leon struggled with command against an Astros team that leads the American League in walks and is second-lowest in strikeouts. He did not have a strikeout, and registered just two swings and misses in the 26 strikes he threw. Eight of those strikes were fouls.

“Ponce is facing a team that’s very disciplined,” Cardinals manager Mike Shildt said. “You need to be in the strike zone and keep them honest. You can expand, there are a few guys you can expand on, but he wasn’t able to control counts.”

Ponce de Leon said he’s looked at video and hasn’t found anything glaring. He felt good every time the Cardinals called him up for a spot start in the first half of the season, but he’s struggled since earning the rotation spot.

“It’s very frustrating,” he said. “I started off very well, and I had a good week in between these last two starts. I thought I was going to come out grooving. But it just didn’t show.”

Correa’s homer was the only run Wacha allowed in the longest relief outing of his career (4 2/3 innings), but the Cardinals deficit was still a large one, despite Paul Goldschmidt’s team-record-tying sixth home run in as many games.

“I felt good throughout the whole night,” Wacha said. “Felt like I was throwing quality strikes down in the zone, getting ahead of guys. Just wish I could have had that pitch back. Just doing whatever [the Cardinals] need me to do, try to go out there and have success right now.”

Ponce de Leon took over for Wacha in the rotation because of Ponce de Leon’s efficiency -- in five spot starts this season, the rookie right-hander had a 1.99 ERA -- and Wacha’s lack thereof. But the question mark with that spot and who inhabits it has returned.

“It’s too early to answer that, but I understand the question,” Shildt said. “I don’t have an answer for you.”

If anything, Saturday night highlighted a potential hole St. Louis could fill at Wednesday's Trade Deadline. The Cardinals have said they’re prioritizing relief help, but a frontline starter could help them just as much, if not more, as they fight for the division, which is still up for grabs. Despite the loss Saturday, the Cardinals stayed in sole possession of first place thanks to the Brewers’ win over the Cubs.

Internally, the Cardinals could switch roles for Ponce de Leon and Wacha again. Otherwise, they have limited options. Their Major League-ready pitching depth has faded throughout the season. Alex Reyes is still on the injured list in Triple-A Memphis with a right pectoral strain. Austin Gomber is on the injured list, too, with a biceps injury, and he’s had shoulder fatigue recently to set him back. Carlos Martinez is the Cardinals’ closer with Jordan Hicks recovering from Tommy John surgery.

Right-hander Jake Woodford turned heads in spring and continues to do so this season in Memphis, but on Friday he allowed 10 runs in 4 1/3 innings. Left-hander Genesis Cabrera struggled when he replaced Wacha in the beginning of June, and he has a 6.42 ERA in 14 games with Memphis this season.

“It’s up to [the Cardinals],” Ponce de Leon said. “I’m going to put in the work I need to stay. … I need to figure out something.”