When the first pitch of what was supposed to be his final appearance in a baseball game this year hurtled toward home plate, Stephen Strasburg sat with a warmup jacket on his back, a hat on his head, in the Washington Nationals’ dugout.

The pitch, an 89-mph fastball that sailed wide of home plate for a ball, was thrown by John Lannan, a pitcher with only a fraction of Strasburg’s ability but, beginning Wednesday night, a distinct advantage over the Nationals’ ace: Lannan can participate in Washington’s pennant race, and Strasburg can only watch it.

This was supposed to be Strasburg’s finale, Wednesday at Citi Field against the New York Mets, his 29th and last start of the Nationals’ turnaround season. But that was before Manager Davey Johnson brought a premature end to that already premature end to what had become not only the dominant issue in Washington’s clubhouse, but across baseball. Strasburg, Johnson determined last Saturday, would pitch no more.

That ending, however abrupt it seemed, deviated in no way from the Nationals’ plan since before the season. Strasburg and his right arm — an arm that was surgically repaired on Sept. 3, 2010 — would be treated with caution that was both extreme and, given the club’s position atop the standings, unprecedented.

But Wednesday represented the plan in its stark reality: Lannan, a soft-tossing left-hander who started twice on opening day for the Nationals in their sorriest seasons, was on the mound, and Strasburg, a flame-throwing right-hander, in the dugout.

“Of course he’s been pestering [pitching coach Steve McCatty] about what he can do,” Johnson said. The conversations, Johnson said, have gone something like this.

Strasburg: “Can I keep throwing off the mound?”

McCatty: “No.”

Strasburg: “Can I play catch in the outfield?”

McCatty: “Yes.”

“It’s pretty bad now, but it’s going to get worse,” Johnson said, “because he’s going to be champing.”

As a means of reaching out to his star-turned-extra, Johnson joked with Strasburg before the game to keep taking batting practice despite the shutdown. “If I can’t use your arm,” Johnson said, “I’ll use your bat.”

But what, exactly, Strasburg is supposed to do with his time appears unclear. When he walked into the clubhouse just past 4 p.m. — wearing a tan suit and enormous black headphones — he headed to the back right corner of the room, plopped down at his locker, and pulled out a laptop computer. A starting pitcher’s life, on days he doesn’t pitch, can be sedentary. But here was Strasburg entering into the final 20 games of the season with neither a debilitating injury nor a stated purpose.

“It’s tough,” third baseman Ryan Zimmerman said. “But what should we say to him?”

On Monday, Strasburg walked into the Washington clubhouse to find the MLB Network pumping on the TVs. As if to greet him upon arrival, the announcer was saying, “. . . and the Nats begin life without Stephen Strasburg.”

“Two seconds?” Strasburg said. He rolled his eyes and quickly shook his head.

Wednesday, the TVs were off when he got to the ballpark to begin another day of . . . what, exactly? His No. 37 jersey hung in his locker above one pair of sneakers, one pair of spikes and one pair of flip-flops. Videos danced across his computer screen. His blazer remained on his back, his headphones on his head. He spoke only when spoken to — when fellow pitcher Jordan Zimmermann rolled his chair over for a few words and then wheeled back, when outfielder Michael Morse ambled over for a joke.

Fifteen minutes after Strasburg arrived, Lannan bopped in, sunglasses on his head, smiling, and walked to the back left corner of the clubhouse, opposite Strasburg. As Lannan dressed for his third major league start of the season, he and Strasburg chatted briefly. Johnson said he will avoid the game so many others will embrace: Look at Lannan’s production — which began with 52 / 3 innings of scoreless ball in a 2-0 win Wednesday — and wonder what Strasburg would have done in his stead.

“I don’t go there,” Johnson said.

By 4:35 p.m., Strasburg’s computer was away, a red Nationals cap sat on his head, his sneakers were on his feet. Not 20 minutes later, he walked onto the field, holding his black glove. He headed to the left field corner, and when the other pitchers — those whose seasons are still alive — began to jog, Strasburg joined them.

“He is now under a rehab program as we speak today, diligently going after it, preparing for 2013, which he couldn’t do if he was pitching,” Scott Boras, Strasburg’s agent, said this week. “So he gets an extra two months of strength and conditioning on his body and flexibility on his elbow to prepare him to pitch more innings without fatigue next year.”

This is the year, though, that the Nationals have the best record in baseball — a record Strasburg contributed to by winning 15 games, a record he can neither help nor hinder from here.

“People act like it’s automatic, like we’re completely giving up our shot to have a deep run in the playoffs because we’re shutting down Stephen,” Zimmerman said.

It is not, Zimmerman said, the way the Nationals think. But on Wednesday night, the portion of the Nationals’ season that had been, heretofore, only talked about began in full. The central character watched another pitcher take his turn on the mound, then lined up to congratulate his teammates afterward, only tangentially a part of it all.

Adam Kilgore and Rick Maese contributed to this report.