In the visitors’ dugout Sunday afternoon at Busch Stadium, John Lannan watched the carnage and waited his turn. A long summer spent in ballparks from Columbus to Charlotte, making starts that were decidedly out of the spotlight and could have seemed irrelevant, has somehow come down, for Lannan, to this: Monday night, he will get the ball at Nationals Park against the Philadelphia Phillies. If he wins that start, the Nationals will win the National League East.

“Just because he spent most of the year in Syracuse,” reserve outfielder Corey Brown said, “doesn’t mean we don’t have confidence in him.”

For players who have been in the major leagues, the difference between Syracuse and Siberia is scant. The Class AAA International League means playing against the Mud Hens (Toledo) and the Iron Pigs (Lehigh Valley). It means bus rides and cheap hotels. Before a shell game left him without a job in the majors — and before he asked for a trade, then struggled when his wish wasn’t granted — Class AAA wasn’t a consideration. It can be where fringe major leaguers go to finish out their careers, slipping away into the abyss.

“It was unfamiliar,” said left-hander Zach Duke, a major leaguer since 2005 before ending up in Syracuse this season. “But he made it out the other end of the tunnel.”

The light at the end of that tunnel will be brightest Monday night. By this point, Lannan is no longer Stephen Strasburg’s replacement, though that is how he will be perceived nationally, because it is Lannan who took the ace’s spot in the rotation when the club followed through on its plan to protect Strasburg’s surgically repaired elbow by shutting him down. Might Strasburg have had the opportunity to pitch the clincher? Sure. But for the Nationals, it is long since a non-issue.

Lannan has twice been an opening day starter for the Nationals, and it was his assumption and belief that if, some day, this woebegone franchise reached the postseason, he would be solidly entrenched in the rotation. But he fell behind Ross Detwiler in spring training, and Lannan found himself in the kind of uncomfortable conversation that happens before the team heads north: There are six players for five spots, and you’re going to the minors.

“You look him in the eye and say, ‘I’m sorry, but these are the five best starters,’ ” General Manager Mike Rizzo said earlier this month. “ ‘But sometime we’re going to call on you to pitch meaningful innings for this ballclub. I guarantee it.’ ”

Outfielder Jayson Werth, at the time, told him, “There’s going to come a time this season we’re going to need you.”

The need is heightened Monday night. Because they lost to the Cardinals, 10-4, on Sunday, the Nationals must win one of their three remaining games — or have Atlanta lose one of its final three — to clinch the division title. As he dressed in a quiet Nationals clubhouse Sunday evening, Lannan politely declined to talk about the next day’s opportunity, not unusual for starting pitchers, who can be of the finicky sort.

But the other Nationals who gathered themselves after dropping two of three in St. Louis professed to have confidence in the lefty, by now a veteran who has not only endured the battles in the majors, but battled back from the bushes to endure them again.

“How could we not?” shortstop Ian Desmond asked.

“What’s he, 4-0?” asked Detwiler, Sunday’s starter and loser. “There you go.”

If anything, since being asked to make two spot starts during the summer and three since he took Strasburg’s turn, Lannan has validated Rizzo’s plan. He went 9-11 with a 4.30 ERA at Syracuse, where he has admitted the mental adjustment was difficult.

“It’s a tough situation for him,” Brown said. “It wasn’t like he went down there and just gave up, but you go from playing in the bigs, you go from playing in D.C. with thousands and thousands of fans, to Syracuse, where you have maybe 1,000 a night. That’s a tough pill to swallow, when you’re so used to being up here.”

Lannan had one poor start, the nightcap of a doubleheader against Los Angeles in which he gave up six runs in 32 / 3 innings. His other four efforts, all of them wins, have been clean; he has given up no earned runs once, two earned runs twice and three earned runs once. Last week, against the same Phillies he’ll face Monday, he allowed two runs in 51 / 3 innings. Even though he got the win, he deserved an even better fate, because shoddy Nationals defense contributed to both runs.

Still, an effort like that Monday, and he could be wholly back from Siberia, or Syracuse, or wherever he spent most of the summer. He could be sipping champagne in a major league clubhouse, directly in the center of the baseball universe.