Prepared to spend the day as a spectator, Tanner Roark woke up Thursday morning and saw Steve McCatty had called him. He had no idea why. Roark would not make his season debut until Friday back home at Nationals Park. What could his pitching coach want? And why couldn’t it wait until he arrived at Citi Field?

Starting pitchers operate an intricate, five-day plan, repeated over and over. They never hear, and typically do not react well to, the three words McCatty had to tell Roark: You’re starting today.

Scheduled starter Jordan Zimmermann ran a fever and stayed up most of Wednesday night with 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. Roark overcame a 31-pitch, two-run first with five scoreless innings, a clutch spot start that spared a bullpen that may see a lot of work during the home opener.

As Roark suppressed the Mets, the Nationals’ offense wore down right-hander Zach Wheeler before against 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. Ryan Zimmerman swatted four hits, including a mammoth homer in the second, and Danny Espinosa laced two opposite-field doubles with a pair of eye-opening swings.

Ross Detwiler made his season debut with two innings, an indication he will not be used as Friday’s starter in the event Zimmermann does not recover in time. Closer Rafael Soriano also made him debut, pitching the final inning of a six-run game.