For a summer’s worth of fun, the sturdiest, most steadfast part of the Washington Nationals was their built-with-ballistics starting rotation. Stephen Strasburg gave way to Gio Gonzalez. Each could blow you away, and each reached the All-Star Game. Think there’s a rest? Here came Jordan Zimmermann, a third starter who throws 95-mph heat. The back end: Ross Detwiler, a former first-round pick, and Edwin Jackson, a player for whom contenders had traded in the past.

Opponents talked repeatedly about the relentless feel of the Nationals. If there’s a backbone to their 98-win season, it’s those five men. So it was stunning Monday afternoon, in the creeping shadows of Busch Stadium, to see one vertebra visibly crumbling, a symbol of the shakiest period Washington’s starters have had all year.

Zimmermann not only lost Game 2 of the Nationals’ National League Division Series against the St. Louis Cardinals, but he was blistered. His three-inning, five-run outing laid the groundwork for what became an emphatic 12-4 defeat.

“You have to pitch,” Nationals Manager Davey Johnson said. “You can’t go out there and just throw against this ballclub. This is a good fastball-hitting club. You have to make some good pitches.”

Zimmermann did not. In a vacuum, this might not be cause for too much concern. The Nationals still head home for Wednesday’s Game 3 tied, and they will send Jackson — who was good enough to be in St. Louis’s rotation a year ago, helping the Cardinals to a World Series title — to the mound. Detwiler, who won 10 games, will follow in Thursday’s Game 4, now assured. The outings by Gonzalez, who would pitch Game 5, and Zimmermann, who is done for this series, are memories.

“The last two starts haven’t been good for us,” Zimmermann said. “We got Edwin going on Wednesday, and he’s going to go out there and send a message.”

The message, thus far, has differed from the one the Nationals sent all year. Zimmermann’s start Monday continued a worrisome trend, particularly against the Cardinals. Strasburg is, of course, no longer pitching. And because the Nationals played in St. Louis during the final weekend of the regular season, they have compiled some recent disturbing data against their playoff opponent.

Mix in poor starts from Jackson and Detwiler in blowout losses Sept. 28 and 30, respectively, with the shaky outings from Gonzalez and Zimmermann to open the postseason, and the Nationals’ most recent turn through the rotation against St. Louis has yielded 112 / 3 innings, 16 walks and a 13.89 ERA. Gonzalez opened the playoffs by tying a career high with seven walks. Zimmerman followed with the shortest of his 33 outings this season.

“They’re not going to have Grade A stuff every single night,” second baseman Danny Espinosa said. “You can’t expect a guy to do that every single time.”

Monday, Zimmermann opened the second inning by allowing four consecutive hits, each seemingly scalded harder than the last. When Allen Craig hit a two-out, solo homer in the third, the Cardinals led 5-1, and Zimmermann was all but done.

One start does not make the series a mess, but there is a trend here. The Nationals’ rotation announced its presence in April, when its ERA was a stunning 1.78, best in the majors. Through the summer, it never had an ERA for a full month that was higher than 3.91 (May). For the year, the starters’ 3.40 ERA was the NL’s best.

In September and October, however, the Nationals’ rotation — including two starts from Strasburg and four from his replacement, lefty John Lannan — ran its ERA up to 4.20, 11th-best in the NL.

Capping those outings: The disasters authored by Jackson and Detwiler as Washington tried to close out the division in St. Louis. Jackson went first, and he could get only four outs before he gave up nine runs, eight earned, in what became a 12-2 loss. Detwiler faced the Cardinals with the Nationals’ first chance to clinch the division without help from anyone else. He failed miserably, lasting 21 / 3 innings while giving up seven runs, though only three were earned.

“I think you can throw that away, definitely,” catcher Kurt Suzuki said.

In the quiet of the visiting clubhouse Monday night, Suzuki and Zimmermann changed into athletic shorts and flip-flops and met in a corner. They broke down the start, bit by bit. Across the room, Jackson — who has pitched in seven postseason games — showered and got dressed.

“Things can get fast in the postseason,” Jackson said. “They can speed up real quickly. Especially as a starting pitcher, you have to control the game. You have to control the pace.”

In the playoffs, the Nationals — between the jitterbugging Gonzalez and the flat-lining Zimmermann — have not done so. It is, by far, too soon to say that a strength has flipped into a weakness. Jackson, after all, threw eight innings and allowed no earned runs against the Cardinals on Sept. 4 in Washington.

But on the off day before Game 3, there is one more painful reality to consider. Two days earlier, in a 4-3 victory over St. Louis on Sept. 2, the Nationals got six dominant innings of two-hit, nine-strikeout, shutout ball from their starter. His name: Stephen Strasburg.