Early Thursday morning, the Roarks of Wilmington, Ill. loaded into their car and started the drive to Washington, where on Friday they would watch their son, Tanner, start the Washington Nationals’ home opener. About the time they reached the Indiana border, Roark arrived at Citi Field, prepared to spend the afternoon as a spectator.

Starting pitchers operate on an intricate, five-day plan, repeated over and over. They never hear, and typically do not react well to, what pitching coach Steve McCatty had to tell Roark at about 10 a.m., three hours before Thursday’s first pitch: “You got the ball.” Roark reacted with a shrug. “Okay,” he said. The only obstacle came when he received a text from his mom that read, “What happened?”

Scheduled starter Jordan Zimmermann ran a fever and stayed up most of Wednesday night because of a nasty stomach virus. When he showed up to Citi Field sick, the Nationals sent him home and handed the ball to Roark. In an 8-2, sweep-sealing victory over the New York Mets, neither the surprise assignment nor a strenuous first inning fazed him.

“I just go with the flow,” Roark said. “Whatever they tell me to do, that’s what I’m here for. If they want me to pitch whenever, I’ll pitch then. It’s basically up to them. I just get the ball and go.”

The Nationals lost their cleanup-hitting catcher for at least a month, skipped an all-star right-hander because of illness and fell behind in the first inning every game. They will still roll into their first clash against the Atlanta Braves undefeated, with a bullpen that struck out 18 in 10 innings, an offense that scored 22 runs and a clubhouse brimming with confidence.

“It shows you how good the starting pitchers are,” said Ryan Zimmerman, who went 4 for 5 with a mammoth home run in the second. “They can give up a couple early runs and then keep them there for six, seven innings. It says something about how they are. We can continue to grind out at-bats and score some runs here and there and have a big inning. That’s kind of what we did.”

Roark overcame a 31-pitch, two-run first inning and finished with five scoreless innings, a clutch spot start that spared the bullpen a day before it may leaned on for an entire game. As the Nationals packed up Thursday evening, Manager Matt Williams said, they still did not know who will start the Nationals Park opener.

The Nationals hope Zimmermann will recover and take the ball. If not, they will piece together nine innings from their bullpen. Ross Detwiler, who had a starter’s workload the first half of spring training, threw two scoreless innings Thursday. In the event Zimmermann cannot pitch, the likeliest starter would be Craig Stammen.

“We don’t want to send him out there with him being really sick,” Williams said. “So we’ll see how that goes in the morning.”

As Roark suppressed the Mets, the Nationals’ offense wore down right-hander Zach Wheeler before again thrashing New York’s bullpen. The Nationals took the lead with two runs in the fifth, then battered relievers for four runs in the seventh. Denard Span reached base three times and tied the score with an RBI single in the fifth, Adam LaRoche drove in three runs and Danny Espinosa laced two opposite-field doubles with a pair of eye-opening swings.

Though the Nationals pounded 13 hits and drew five walks, Roark provided the most crucial performance. He allowed two runs over six innings on six hits and three walks. He needed only 64 pitches for his final five innings. After he allowed a one-out double in the fifth, with the Nationals leading by only one run, Roark struck out the final four batters he faced.

And he thought he would be watching. The Nationals trusted Roark to handle the start, a confidence he earned in call-up last season. Roark went 7-1 with a 1.51 ERA and helped propel the Nationals’ late-season surge. No situation rattled him; he hardly even changed facial expressions. In the winter, when McCatty talked to him about his role, Roark replied, “I don’t care what I do. Start, bullpen, whatever. Tell me when, I’ll be ready.”

When Roark walked into the clubhouse, McCatty told him he might be starting. “All right,” he thought. “What’s going on?” Roark saw Zimmermann enter the room, and “he looked like he was pretty sick,” Roark said. A few minutes later, McCatty informed Roark he would start.

Roark needed no last-minute cramming about the Mets. He had watched the series’ first two games closely, and he had a good idea of how he wanted to attack them.

“I had a couple hours to get ready,” Roark said. “It’s surprising, but nothing you can do about it. So I went out there and did my thing.”

Said McCatty: “It’s only as difficult as you make it. He just has the attitude to do that, so it was no problem.”

Roark’s odd start began with trouble, a first inning that hinged on the fly ball to shallow center that Span lost in the sun. After the two-run, 31-pitch first, Roark settled. He moved his sinker to both edges of the plate. He took advantage of a Mets lineup that, having struck out 31 times in the series’ first two games, hacked early in counts. Roark worked around a leadoff single in eight pitches in the second inning, and in a 1-2-3 third inning he threw only seven pitches.

“He spotted the ball,” McCatty said. “He did what he does.”

Roark pitched with the same efficient style that fueled his ascension last year. He stayed cool on the mound. He located his 92-mph fastball to both sides of the plate and twirled his curveball for strikes. He mostly got ahead in counts, throwing 16 first-pitch strikes to 26 batters.

“For him to step up and do that, it was awesome,” Espinosa said. “He’s aggressive. He throws strikes. He’s in the zone. He goes after hitters. He doesn’t shy away from anybody. He just makes good pitches. He doesn’t miss too often. It’s fun to play behind Tanner. He keeps a good rhythm in the game.”

The Nationals could relax as Rafael Soriano closed the game. Before it began, they briefly didn’t know who would start. They just knew they could count on Roark.