CHICAGO — When Nationals Manager Dusty Baker said Tanner Roark would start Game 4 of the National League Division Series, jaws dropped. Tuesday’s rainout seemed so sure to help the Nationals, allowing them to replace Roark with Stephen Strasburg, dramatically increasing their chances of survival. How, with the season on the line, could the Nationals’ hottest pitcher sit out a must-win game?

“Didn’t work out for Stras for his bullpen day, and we’re all creatures of habit,” Baker said in his news conference after the game was called. “… Stras, it would have been better because he’s feeling under the weather like a lot of my team is.”

Asked to clarify the comment about his bullpen session, Baker said Strasburg had thrown Tuesday, which would have precluded him from throwing Wednesday. A reasonable question followed: How could the Nationals let that happen?

They didn’t. The team clarified later, explaining that Baker misspoke. Multiple people familiar with the situation confirmed the team’s message, that Strasburg had, in fact, thrown his bullpen Monday. Strasburg was in the dugout for Monday night’s game, and played catch in the outfield at Wrigley Field on Tuesday.

But if he threw his bullpen Monday, Strasburg, a stickler for routine, would have been able to pitch Wednesday. So why won’t he?

Baker told reporters Strasburg was “under the weather.”

“A lot of my team is under the weather with the change of weather and the air conditioning in the hotel and the air conditioning here,” said Baker, who added the team had to change hotels Tuesday after the rainout because theirs didn’t have enough rooms to accommodate them for the extra night in Chicago. “It’s just this time of the year for mold around Chicago — I think it’s mold. I mean, I have it, too.”

When the team clarified its story later, the message was that Strasburg was too sick to start and his endurance was down in his bullpen session. Under those circumstances, the team said, Roark gave them the best chance to win.

People close to the situation confirmed that Strasburg is indeed sick. One used colorful adjectives to describe the extent of the illness. When asked if he could pitch Wednesday, according to a person familiar, Strasburg said he “would give [the Nationals] what he had.” Those involved in the decision — Baker, General Manager Mike Rizzo, Strasburg and pitching coach Mike Maddux — concluded what he had would not be much.

Strasburg’s health has emerged as a limiting factor time and again in his Nationals career. The 2012 shutdown that kept him from participating in the postseason that year was not his decision. But he has landed on the disabled list eight times in his eight-year career, including twice last season, the second time with a torn pronator tendon that cost him the 2016 postseason. Earlier that season, Strasburg was lined up to pitch against Clayton Kershaw in a nationally televised game before becoming a surprising late scratch with upper back trouble. He also landed on the disabled this season, in late July, when a nerve impingement in his right elbow cost him three weeks.

At the time of that injury, Strasburg posited that going to the All-Star Game, where he didn’t pitch, disrupted his routine. The change in rhythm and interrupted treatment could have led to the elbow trouble, he noted, without saying for sure. Strasburg is a relentless adherent to his routine, which is why Baker’s initial story about throwing a bullpen Tuesday made some sense. Obviously, that story was inaccurate, and his between-starts routine was not the issue.

Instead, his health seems to be limiting the Nationals’ homegrown ace once again. Entering this postseason, Strasburg had appeared in one postseason game in the Nationals’ three prior trips. He threw five innings against the Giants in 2014.

But handed all the pressure of Game 1 this season, Strasburg responded with seven innings in which he struck out 10 and surrendered two unearned runs. He took the loss, but looked like a man ready for the moment. Perhaps he will be ready again in Game 5. But with the season on the line Wednesday, he will be the only Nationals pitcher not available. They might not get to Game 5 without him.

Max Scherzer, meanwhile, marched into Baker’s office Tuesday morning and announced that he would be available for an inning in Game 4 if needed. He probably won’t be able to give the Nationals much after throwing 98 pitches in Monday’s Game 3 loss. But a person familiar with the situation said the Nationals will be willing to take Scherzer up on his offer.

Gio Gonzalez, who will be a day short of regular rest, will be available for multiple innings. If the Cubs start a left-handed-heavy lineup against Roark, the Nationals can bring in Gonzalez and perhaps entice Joe Maddon to burn some of his players to reconfigure his lineup accordingly. Then Scherzer, Ryan Madson, and Sean Doolittle could be available for key situations. In other words, the Nationals will be all hands on deck Wednesday. If they win, Strasburg will start Game 5 on Thursday night in D.C., with the hope that extra day will allow him to regain at least some measure of health.

Roark finished his regular season with two rocky outings and hasn’t started a game since Sept. 27. In that start, he allowed six runs on seven hits and five walks in 4 2/3 innings against the last-place Phillies. He made his final regular season appearance in the finale out of the bullpen against the Pirates. He surrendered two runs in one inning.

“We have full confidence in Tanner,” Baker said.

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