Jonathan Papelbon is 9 for 10 in save chances. (Photo by Katherine Frey/The Washington Post)

KANSAS CITY — Think back to spring training, when the Nationals bullpen was an unknown with many new faces and several possibilities. Yet, here we are, 25 games into the season, and those spring days of bullpen question marks feel distant.

The Nationals bullpen of Jonathan Papelbon, Shawn Kelley, Oliver Perez, Felipe Rivero, Yusmeiro Petit, Sammy Solis (swapped in for injured Matt Belisle) and Blake Treinen has been the best in the National League.

“What’s most important is that all of us knows the situation we’re going to pitch,” Perez said. “We have great communication and we prepare well. Everybody is enjoying what they’re doing.”

Entering Monday’s game against the Royals — a team built, in part, on the strength of its relief corps — the Nationals bullpen had the best ERA in the National League. With another dominant performance — three innings and only one base runner — the Nationals bullpen’s totals improved even more. They have a collective 2.48 ERA — best in the NL but sixth overall — and have struck out 73 batters in 72 2/3 innings. Papelbon is 9 for 10 in save chances.

The bullpen “was questionable by you guys [the media]; it wasn’t questionable by us,” Manager Dusty Baker said. “We just didn’t know where we were going to slot the guys. That was the only question. So we found out rather quickly where we were going to put guys and how we should use them and how many days. You don’t know guys and Mike didn’t know guys. We confer a lot in the dugout on which guys are available before the game and during the game. We have some interchangeable parts. The guys are feeling confident. Confidence is the key.”

The Nationals bullpen undermined their 2015 season. When performance was poor or relievers got hurt, the Nationals had limited options, even dipping down to Class AA Harrisburg to call up arms. This offseason, the Nationals shopped in bulk, amassing good but not flashy or top-line relievers.

As a result, the depth improved and Baker had more options. So now when Baker can’t get a seventh inning out of Gio Gonzalez on Monday, he can turn to Solis — instead of Rivero, who pitched the night before — to bridge to the back of the bullpen with mid-90s fastballs and improved command over last season.

“A great group of guys down there,” Solis said. “We have a lot of fun but I think it’s also a veteran staff, excluding myself and maybe Blake Treinen. These guys know what they’re doing and I pick their brains constantly and we’re on top of it right now. It’s exciting and it’s fun to be down there.”

After Kelley gave up a double and got a strikeout in the eighth against right-handed batters, Baker turned to Perez to face the left-handers. So far, the relief parts have been interchangeable and versatile.

“That’s big over the course of the season because you keep them fresh and don’t wear out guys,” Baker said. “On the other hand, guys rust out versus tire out. It’s early in the season and can’t just ride guys and jump to somebody else and then somebody else. This is a full-team effort and guys realize that. And the next man [up], that’s the kind of attitude these guys have. I didn’t do it this time but he picked me up. Or the next time, you didn’t do it and I’ll pick you up.”

The Nationals aren’t a perfect team. Their lineup has been inconsistent and, on most nights, just enough. They have been mostly healthy so the Nationals roster depth has yet to be tested. There is plenty of time to see how the Nationals respond to the adversity the season will bring. But the Nationals have already displayed a quality of good teams: They have won in spite of their weaknesses.

“When we left spring training, I said it usually takes your offense some time to get going,” Baker said. “When hot weather gets here, that’s when the offense generally gets going. Early in the season, like we talked about in spring training, you really rely on your bullpen because your starters aren’t really ready to go more than six or seven innings. Your bullpen is so very very important throughout the year but really important in the beginning of the year.”

Unlike last season, there haven’t been any quiet rumblings yet about reliever usage. Baker has straddled innings with relievers. He has played the match-ups, even with veteran relievers such as Papelbon or players on lucrative contracts such as Kelley. The results and wins have piled up. The Nationals bullpen will have many hurdles to clear over the long season, but so far, it has been surprising.

“A band of brothers out there,” Gonzalez said. “They’re fighting for every inning out there. You want to make sure you keep the game as close as possible and let them do the rest. I love the way they’re going about it, especially the way Pap is shutting it down. Like I said, he’s a great closer. He’s been doing it for a long time. He’s gonna continue to do that.”