Sammy Solis reacts after giving up a ninth-inning grand slam. The Nationals lost, 11-8. (John McDonnell/The Washington Post)

As soon as the bat connected, the sound splitting through the air like a firework, Austen Williams spun to watch the baseball sail far beyond the left field fence.

The hitter was Patrick Wisdom, and he had just smacked the St. Louis Cardinals’ third home run in 11 pitches. The pitcher was Williams, a 25-year-old making his second major league appearance, laboring through a sixth inning he started but could not finish in the Washington Nationals’ 11-8 loss Tuesday night at Nationals Park.

Here, in Williams, was what the Nationals (69-70) are inviting this September, when rosters are expanded. They want to test their young players and take stock of the future. That may drum up excitement, especially in a season defined by how far the team is from the playoff race with 23 games left to play. It may also bring growing pains.

Four hours before first pitch, and six hours before the Cardinals belted those park-deadening home runs to open up a tie game, the Nationals’ clubhouse buzzed with activity and anticipation. There was a regular visit from the team barber. There was pitcher Kyle McGowin, called up for the first time at 26 years old, his face still showing disbelief as reporters approached and Koda Glover yelling “fresh meat!” in his direction. There was starter Erick Fedde, in sandals and shorts, his long legs gliding him from the kitchen to the field and, soon, to an anticipated return from injury.

And there were Victor Robles and Juan Soto, two of the Nationals’ most promising young players, linked for what they are and what they could be in years to come. The 21-year-old Robles made his major league debut last September, and he was added to the roster Tuesday along with Fedde, McGowin, pitchers Joe Ross and Austin Voth and infielder Adrian Sanchez.

Nationals right-hander Eric Fedde gave up four runs and struck out seven batters in five innings of work. (John McDonnell/The Washington Post)

Robles’s locker was situated next to that of Soto, the 19-year-old rookie who is hitting .299 with 16 home runs. They laughed together at their two stalls, Robles’s feet tapping the clubhouse carpet, Soto pulling on his batting practice shirt. They talked together in the dugout, Robles fiddling a glove onto his right hand, Soto squeezing the handle of a wooden bat.

Then they ascended the dugout steps, one after the other, and it was easy to picture them playing in an outfield together, too.

“We’ve always talked coming up like, ‘Let’s play at this level together,’ and then, ‘Imagine us playing at that level together,’ ” Robles, who later flied out to center in a pinch-hit appearance, said before the game through team interpreter Octavio Martinez. “And now that we’re here, it’s a little bit of a dream come true for both of us.”

Fedde was the Nationals’ top pitching prospect heading into this season and joined the rotation early before landing on the 60-day disabled list with a shoulder injury. His first major league outing since July 4 started with two strikeouts in a breezy first inning, his sinker humming at 95 mph, his slider darting in and out of the zone.

Then it was stained by Marcell Ozuna’s solo home run to start the second. Then it was spoiled by Matt Carpenter’s single in the same frame that drove in three runs after Adam Eaton bobbled the ball in right field. Then Fedde recovered, if only a little too late, by blanking the Cardinals in the third, fourth and fifth before he was lifted from the game after 91 pitches.

Outside of the second, Fedde faced 13 hitters, walked one, gave up no runs and no hits. But his final line included four runs and four hits from that rocky inning, and those count, too.

The Nationals’ offense, after sputtering to start, responded with patience and power in the fifth. After Ryan Zimmerman reached on an error, rookie catcher Pedro Severino went down 0-2 and blooped a one-out single on the 10th pitch of his at-bat. Rookie outfielder Andrew Stevenson fell behind 0-2 and worked a seven-pitch walk. Trea Turner walked on nine pitches two batters later to plate Washington’s first run, and Bryce Harper drove in two more with a scorching double to left-center. After an intentional walk to Anthony Rendon, Soto worked a bases-loaded walk to tie the game.

But then the Cardinals jumped on Williams right away, as Ozuna, Paul DeJong and Wisdom all sent four-seam fastballs into the stands. Williams was sharp in his debut Sunday, using a mid-90s fastball and snapping curve to work through two scoreless innings. He couldn’t avoid bats Tuesday and issued a single and walk after the three home runs before Manager Dave Martinez came to get him.

Fans had been shouting for Williams to be removed before the inning spiraled out of control, and they made it known when Martinez eventually walked to the mound.

Later, Yadier Molina’s ninth-inning grand slam off Sammy Solis gave the Cardinals an 11-5 lead and proved to be the difference when the Nationals rallied for three runs in the bottom of the inning and had the tying run at the plate when Matt Wieters grounded out to end the game.

There were more glimpses of the future Tuesday night, more bodies filling the dugout, a few more reasons — like Fedde’s better innings, or Soto’s patience, or that brief Robles sighting — to believe that this down season will not spill into the Nationals’ future.

Yet there was also a reality, that looking ahead can be frustrating, that this September, unlike the few before it, holds doses of both headache and hope.

Correction: An earlier version of this story included an incorrect last name for the Nationals’ translator Octavio Martinez.