The Cardinals’ pitching depth, already run thin, has taken another hit.

Left-hander Kwang Hyun Kim left Friday night’s 6-4 loss to the Reds at Busch Stadium in the top of the fourth inning with lower back tightness, signaling to the dugout and then exiting the contest after throwing a single warmup pitch. Kim threw 47 pitches up to that point, tagged for a pair of homers, but he showed no visible discomfort in the game before promptly leaving with assistant athletic trainer Chris Conroy.

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Manager Mike Shildt said that Kim “jarred” something when he tried to beat out a grounder as a batter in the third inning. It’s the same back ailment that sidelined Kim for two weeks in Spring Training, Shildt added, but it’s not thought to be as serious.

“He feels like he's ahead of it relative to the last time it happened,” Shildt said. “So that part's satisfactory, but it obviously precluded him from being able to continue.”

Kim was receiving treatment following the loss and will continue to be evaluated before the Cardinals make any decisions on his immediate outlook. Shildt was encouraged by the fact that the club has two off-days next week, which would allow more time for Kim to loosen up and recover before his next potential start.

It was a concerning development that glossed over even some positive signs for the Cardinals on an overall woeful Friday, with Edmundo Sosa hammering his first career homer, Tyler O’Neill turning in a pair of web gems in left field and the offense putting the winning run at the plate by scoring three in the ninth after being held to a lone run by Reds starter Luis Castillo.

Righty Jake Woodford replaced Kim in the fourth, warming up in full on the mound before tossing two 1 2/3 scoreless innings, albeit walking two and plunking a pair. He’s just one pitcher whose role will likely elevate now that Kim can be added to the list of hurting Cards pitchers, along with Jack Flaherty (oblique strain) and Miles Mikolas (forearm tightness) already on the injured list, as well as other arms down at the Triple-A level. Johan Oviedo is another candidate to step up, and he’ll be Saturday’s starting pitcher as the Cards eye a series split with Cincinnati.

But even before Kim’s setback, the Cardinals had already said they plan to dip into the trade and free-agent market for reinforcements, because Flaherty and Mikolas’ injuries are thought to be more in terms of months than weeks.

As it stands, 39-year-old Adam Wainwright serves as the Cardinals No. 1 starter. Behind him, Carlos Martínez is coming off a start in which he allowed 10 runs in the first inning, John Gant is putting up zeroes, but doing so with poor underlying numbers, and now Kim’s future is uncertain.

Complicating the Cardinals’ flexibility to make moves is the rigidity of their 40-man roster. There’s one open spot, after Tyler Webb was designated for assignment on Thursday, but five other slots are occupied by players on the 10-day injured list and three by players on the 7-day IL in the Minors.

The Cardinals are hopeful Kim’s injury is not one that accelerates immediate -- and simple -- need for innings. Regardless, dealt with the same ailment in Spring Training, he “was locked up” after a bullpen session, Shildt said, and then shut down for almost two weeks, not making his first regular-season start until April 17.

Frustrating, too, is that Kim’s injury was the second for a Cardinals pitcher in less than a week that came while not pitching. Flaherty felt his oblique tightening up while pitching against Los Angeles on Monday and then exited when he further strained it while nicking a foul ball off Trevor Bauer. Kim was sprinting at 28.7 feet/second (well above the Major League average of 27.0) when his left foot hit the first-base bag and ultimately ended his night.