ST. PETERSBURG -- Wednesday would have been Shane McClanahan’s night to pitch, a turn in the Rays’ rotation that all but guaranteed victory the first three months of this season.

But with McClanahan sidelined by a left forearm injury that will likely require surgery -- leaving Tampa Bay’s ace left-hander “highly unlikely” to pitch again this season -- the Rays instead ran an "all hands on deck" bullpen game that went awry early in a 6-4 loss to the Cardinals at Tropicana Field.

Left-hander Jalen Beeks got the start, his first Major League appearance since July 4, and allowed two runs on three hits and one walk while recording six outs. Typically reliable rookie right-hander Kevin Kelly followed Beeks to the mound, and he surrendered three runs on five hits and one walk in two innings. Righty Robert Stephenson allowed a seventh-inning solo shot to Cardinals leadoff man Lars Nootbaar.

The Rays got a pair of home runs from Jose Siri (who has a team-leading 23 on the year) and a 3-for-3 game from Wander Franco that included a Statcast-projected 448-foot solo shot in the seventh inning, the longest homer of his young career. But it wasn’t enough to overcome the early deficit.

“We don't go through a bullpen day assuming that we're just going to shut people down,” manager Kevin Cash said. “Bullpen day or starter, I hope they're on. I want them both to be on.”

The question now facing Tampa Bay: With McClanahan joining Shane Baz, Jeffrey Springs and Drew Rasmussen on the list of key starters unavailable due to injuries, where does its rotation go from here?

Converted reliever Zack Littell will get another starting assignment in Thursday’s series finale. The right-hander has put together a 1.65 ERA with 12 strikeouts and just one walk in 16 1/3 innings over four outings (three starts) since he was slotted into the regular rotation for the first time in his big league career.

“I kind of treat it the same way, honestly,” Littell said Wednesday afternoon. “My role doesn't necessarily change. I think they've been fairly communicative that they just want me to go out there and do what I've been doing, continue to try to eat innings and take it one step at a time.”

Trade Deadline acquisition Aaron Civale is slated to start Friday’s series opener at The Trop against the Guardians, his former team. Veteran right-hander Zach Eflin is lined up to pitch Sunday’s finale after making another strong start in Tuesday night’s series-opening win against St. Louis. And the Rays hope to see reigning American League Pitcher of the Month Tyler Glasnow return on Saturday after being scratched in Detroit on Sunday due to back spasms.

Even with Glasnow healthy, though, the Rays have only four starting pitchers on their roster: Glasnow, Eflin, Civale and Littell.

Nobody can truly replace McClanahan, the two-time AL All-Star who has been among the game’s best when healthy the past two years. But how might Tampa Bay fill the hole in its rotation?

First, it’s worth noting the Rays may have to do so only a few more times this month. They played 109 games through July, more than any other team in the Majors. Their August schedule includes five off-days, with three more still to come.

That means the fifth spot in their rotation may come up only twice more -- assuming everyone else remains healthy and stays on turn -- before a more glaring need for a “true” fifth starter presents itself with a stretch of 17 games in 17 days beginning Sept. 1.

It’s too late to deal for an established arm, as the Trade Deadline passed on Aug. 1. The Rays were involved in talks for a number of veteran starters, preferring the idea of accumulating too much pitching over not having enough, but they crossed the finish line only on Civale.

Instead, the Rays could roll with a couple more bullpen games (with right-hander Erasmo Ramírez among the top candidates to provide multiple innings) or summon a spot starter/bulk-inning pitcher from Triple-A Durham. Their best backup options in the latter role are righty Cooper Criswell, rehabbing left-hander Josh Fleming and perhaps lefty prospect Jacob Lopez.

Whether it’s this month or after rosters expand from 26 to 28 players on Sept. 1, the Rays will likely turn once again to rookie starter Taj Bradley. They sent their former top prospect back to Durham on July 31 after acquiring Civale, wanting him to continue his development and find his groove after struggling through a stretch with a 9.12 ERA in six big league starts.

“For a young pitcher, the last four or five or six starts, the way it's gone has been a challenge,” Cash said recently. “But we need Taj to be really good. That was reiterated to him. He's a really confident kid, and he'll go down there and make the most of it."