Stephen Strasburg (37) talks with Nationals pitching coach Derek Lilliquist (38) during a second-inning mound visit Friday against the San Francisco Giants. Strasburg did not come out for the third. (Geoff Burke/USA Today Sports)

So few moments can alter a baseball season one way or another; so few games can change a team’s fate when teams have so many games to play. But the departure of an ace, the sudden stumble of a stalwart, is one such moment. In a sport that makes so much of minutiae, an injury can qualify as massive.

When Stephen Strasburg did not jog out to start the third inning of Friday night’s 9-5 loss to the San Francisco Giants, his absence lingered. When he conducted his postgame interview, more sullen than normal, his demeanor undermined the message of his opening line.

“I mean, in the grand scheme of things, I’ll be okay,” Strasburg said. “But it’s frustrating. Hopefully we’ll get some answers the next couple days.”

Strasburg left the game with what Manager Dave Martinez described as “a little inflammation” in his shoulder, though nothing about inflammation in that joint, for a pitcher with Strasburg’s injury history, is ever little. He has experienced elbow problems and back problems, Tommy John surgeries and even a dose of shoulder inflammation in his rookie year, learning what to pitch through and what not to in the process. But Strasburg admitted that this discomfort accumulated over weeks.

“I think every pitcher, if you ask them, they’re not going to feel 100 percent every time. I think it’s just been something that’s been a gradual process. It’s been affecting me more and more over the last three, four starts,” Strasburg said. “It’s a tough one to gauge because you want to go out and do your part.”

A man whose toughness has been debated in this city since his debut, eight years ago Friday, was trying to push through. Perhaps he pushed too hard.

“I want to get the results of the MRI [exam] tomorrow before we make any conclusions,” Martinez said. “Hopefully it’s nothing at all, just a little tightness, little inflammation and we can take care of it and we can go from there.”

The night began with promise, with a sellout crowd percolating with Stanley Cup fervor, and in those circumstances, Strasburg’s injury felt like an inevitable dose of reality. The Nationals acknowledged the Capitals with a tribute on the video board before the game. The crowd stood and cheered. The city has changed since the Nationals last played here — two days and championship ago. That difference was not lost on Nationals players, some of whom talked about it before the game. Everyone needs reminders of the possibilities now and then, and sports had given this city so little permission to believe.

But then the game started, and the goodwill succumbed to Andrew McCutchen, who turned Strasburg’s 97 mph fastball around to dead center field to give the Giants a 1-0 lead. Eight years ago to the day, McCutchen was the first big league batter Strasburg ever faced. Both men have traveled winding paths since.

That hit, and the hard-hit balls that accompanied it that inning, became a full-on barrage in the second. The Giants only scored two runs despite four hits and two other well-hit balls, but they looked completely comfortable against Strasburg, who looked entirely uncomfortable against them. He landed off to the first base side, as if trying to avoid the stabs of some pain. He shook and stretched his shoulder.

So often with Strasburg, poor results foretell injury trouble. When Strasburg is right, he dominates. When Strasburg is trying to wrestle through something, he doesn’t. He had allowed at least three runs in six of his last eight starts, and struck out double-digit batters twice in that time. By Strasburg’s standards, he didn’t look right.

His velocity never wavered — just fine, in the high-90s, no cause for concern. His breaking stuff broke, but not where he wanted. Poor command, not waning stuff, seemed to be the biggest symptom of injury.

For a team that just lost its No. 5 starter, Jeremy Hellickson, to a hamstring injury, losing a No. 2 would be cause for serious concern. Erick Fedde pitched Friday, meaning he would be on turn to fill in for Strasburg if needed. His next turn would come Wednesday at Yankee Stadium. But the future remained uncertain as of Friday night.

“I’m trying not to forecast anything,” Strasburg said. “I think I’m just going to get an MRI tomorrow and take it from there.”

Strasburg’s injury presented more immediate concerns for the Nationals’ bullpen, which couldn’t hold the Giants in place, even as the Nationals tried to mount a comeback. Juan Soto pulled them back with a two-run shot to left field against lefty Andrew Suarez, a former Nationals draft pick who is not the first left-hander Soto has victimized with an opposite-field homer.

Bryce Harper, slumping these days, hit a two-run single to pull them within two in the fifth. Matt Adams’s two-out pinch-hit single brought the Nationals within one in the sixth. Little by little, the Nationals chipped away, undermining the Giants’ lead, dislodging the cloud that had settled over South Capitol Street following Strasburg’s departure.

But Wander Suero allowed three runs. Brandon Kintzler, his sinker too strong after five days off, did too. Matt Grace inherited a bases-loaded situation and couldn’t get out of it. This bullpen has been a strength in recent weeks, and one bad day does not rewrite that.

But one day can rewrite a season, even in this sport that covers so many days, if it includes an injury to an indispensable star. Unfortunately for the Nationals and Stephen Strasburg — uncertain of what lies ahead for the annual Cy Young candidate — Friday might just be one of those days.