CHICAGO -- The Cubs have a true ace right now in Yu Darvish. They have a metronome in Kyle Hendricks. Jon Lester, even when having an off night, can lean on his wealth of experience and search for ways to gut his way through outings.

The rest of Chicago's rotation is full of question marks, which is a precarious position for the National League Central leaders as October looms.

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"Today's a nice day to take a look at Adbert, right?" Cubs manager David Ross said before a 4-2 loss to the Cardinals in Game 1 of Saturday's doubleheader at Wrigley Field.

Yes, it was indeed as good a time as any for the Cubs to again see what they have in pitching prospect Adbert Alzolay (No. 6 on MLB Pipeline's Top 30 Cubs Prospects). That said, his trip to Chicago from the alternate training site in South Bend, Ind., was a product of circumstance rather than purely a promotion.

Right-hander Tyler Chatwood is on the 10-day injured list with a right forearm strain and has no timetable for return. Lefty José Quintana -- the arm Chicago hoped to plug into Chatwood's rotation slot -- landed on the 10-day IL on Wednesday.

There is roughly a third remaining in this 60-game slate, and the Cubs need a fifth starter.

Enter Alzolay.

Back on Aug. 19, Alzolay was impressive in a five-inning spot start against St. Louis in a different doubleheader. In that outing, the right-hander held the Cardinals to one unearned run, finishing with six strikeouts and one walk in a no-decision. That helped push Alzolay atop the list of rotation reserves.

On Saturday, Alzolay lasted only 2 2/3 innings and saw his pitch count climb to 78 (43 strikes). He struck out four and allowed just two hits, but he issued five walks and exited with his team trailing, 2-1. It was a mixed-bag showing that was not helped by a couple of defensive miscues.

“I thought it was the first time the youth showed back up a little bit,” Ross said. “He just got a little rushed. I think he was trying to do a little too much at times and got a little out of sync with his rhythm. I think that's all. The stuff's there.”

In the second, Alzolay issued a leadoff walk to Paul DeJong and then fell into a two-on, none-out jam after a fielding error by third baseman Kris Bryant. From there, Alzolay locked in and escaped with two strikeouts (Matt Carpenter and Dylan Carlson) and a groundout (Lane Thomas).

Alzolay was not as fortunate in the third inning.

Alzolay gave up two singles and later walked Brad Miller to load the bases with one out. The righty then induced a grounder off the bat of DeJong and Bryant began what had the makings of an inning-ending, 5-4-3 double play. Second baseman Nico Hoerner's throw was a bit wide and first baseman Anthony Rizzo missed the catch, bringing a run in for the Cardinals.

“I got the ground ball,” Alzolay said. “But it’s part of the game.”

From there, Alzolay issued two more walks -- one to Carpenter with the bases loaded -- before being lifted by Ross. Complicating matters for the Cubs was also the fact that the lineup was mostly quiet against St. Louis righty Adam Wainwright -- save for a pair of solo homers by center fielder Ian Happ.

Alzolay's performance certainly gave the Cubs more to ponder.

“We have a lot of faith in Adbert,” Ross said. “He'll learn from this outing and continue to get better.”

Over the first 16 games of this season, Chicago's rotation went 12-3 with a 2.55 ERA, which was the second-lowest mark in the Majors. In the next 22 games, leading up to Saturday's twin bill, the rotation had gone a collective 6-8 with a 5.15 ERA, and 2-8 with a 6.48 ERA without Darvish's stellar starts.

Lester has an 8.55 ERA in four starts during that stretch, while Alec Mills has a 7.66 ERA in five outings. The Cubs need them to get back on track, while the team considers its options the next time the need for a No. 5 comes around (Thursday vs. the Reds).

“I'm here to help the team in whatever spot they need me, you know?” Alzolay said. “For this year, it doesn't matter if it's out of the bullpen or a spot-start like I've been doing lately. Like I said, that's my mindset right now.”

Alzolay remains the top choice -- with righty Tyson Miller also an option after being recalled between games Saturday, as the club designated Steven Souza Jr. for assignment. Then, there is the question of whether the Cubs would consider promoting lefty Brailyn Marquez, their No. 1 prospect. It might not be out of the question before the season ends.

"It's just a next-man-up thing," Mills said after his most recent outing. "I think we've got enough guys down there that can really come up and pitch and make outs. So, we'll fill in the best we can."