Matt Adams rounds the bases after his eighth-inning grand slam Sunday at Nationals Park. (Katherine Frey/The Washington Post)

Maybe Kurt Suzuki is retired instead of ripping a two-out, two-run single that jump-started the Washington Nationals on Sunday afternoon. Maybe the Nationals don’t bully a bunch of middling relievers. Maybe Aníbal Sánchez, when pushed into the sixth inning, starts to wither, then wilts, then is whisked from the game before it gets out of hand.

Maybe the Nationals don’t win this one a few weeks ago — or a month ago or back when they couldn’t seem to do much of anything without slipping up. But if there has been a change in this team, incremental or otherwise, it again peeked out in a 15-5 victory over the Arizona Diamondbacks at Nationals Park.

The offense was good when it should have been while facing a mash-up of the Diamondbacks’ bullpen, riding two homers from Matt Adams (a career-high seven RBI) and one apiece from Anthony Rendon and Suzuki, who exited in the seventh inning after a bouncing pitch struck him in the neck. Sánchez provided six solid innings, giving up just two runs and throwing 98 pitches, his most since May 5. The bullpen was fine aside from Tony Sipp allowing two runs (one earned) in the ninth before everyone could head home.

“That was a good Father’s Day gift, by the way,” Manager Dave Martinez said. “I thanked all my sons in [the clubhouse].”

But it won’t matter if Washington can’t carry this success into a critical week of National League East home games — four against the Philadelphia Phillies and three against the Atlanta Braves.

By taking Sunday’s finale, the Nationals have not dropped a series since they were swept in four games by the New York Mets on May 20-23. They found a rhythm after those losses in New York, when their season was spiraling away, when their only consistent tendency was not being consistent at all. But some of those concerns have popped back up in the past 10 days or so.

The Nationals have split their past three series, against the San Diego Padres, the Chicago White Sox and the Diamondbacks, and so a 9-2 surge was followed by a 5-5 stretch that slowed their momentum. Patrick Corbin, who will start against the Phillies on Monday, has had three consecutive rough starts. Stephen Strasburg and Erick Fedde weren’t sharp in their outings against Arizona. The offense, before clicking again Sunday, had been slipping in and out of consciousness. The bullpen’s resurgence has hit a few snags.

In the course of a 162-game season, which Washington is now nearly halfway through at 33-38, you can have fits of mediocrity and still succeed. It’s expected. It’s basically required. But the Nationals lost their margin for error with how they played in March, April and most of May. They finished the weekend 8½ games behind the Braves, six behind the Phillies and even one behind the Mets.

And so they need — for real this time — for Sunday’s victory to serve as a springboard.

“We’ve got to win those games. It’s important,” right fielder Adam Eaton said. “We’re trying to chase at this point. Not to put too much emphasis on it, but we need to play some really competitive baseball.”

Taking care of Sunday at first looked as if it would prove difficult. Ketel Marte whacked Sánchez’s seventh pitch over the right field wall to put Arizona up 1-0. Sánchez has been nearly unhittable since he returned from the injured list in late May, pushing his bad start deeper into the past, but he made a few mistakes in this one. That included Marte’s homer, two hits and a run in the fourth and smaller hiccups that he was able to work around for his 100th career win.

Luckily for Sánchez, the offense clicked right away against a collection of Arizona relievers. Eaton had the team’s first of 15 hits and reached base in all five of his plate appearances. Suzuki capped a three-run first inning with that two-out single. Rendon and Howie Kendrick finished with three hits and two RBI apiece, strengthening their all-star bids, and Juan Soto, Rendon and Eaton all scored three runs.

But the effort was paced by Adams and his two towering home runs. The first came in the third, scoring three, and gave Washington a comfortable lead. The second was a grand slam in the eighth that splashed into the upper deck in right field.

Two seasons ago, when Adams was with the St. Louis Cardinals, he and his dad made a deal: Jamie Adams would come to wherever his son was playing each Father’s Day. So Jamie made the three-hour drive from central Pennsylvania to watch Matt smack one baseball 438 feet and another 399.

“It means a lot,” said Adams, who has three homers in the past two games, “especially having my dad in the stands.”

It was, by afternoon’s end, another complete win in a month that has included several. But the start of the two most important series of June — and the season to date — arrives Monday. Maybe the Nationals can use them to take their season back. Or maybe that will be too much to ask.

“We’ve got to take care of tomorrow,” Martinez said Sunday, sticking with his one-day-at-a-time message. “Obviously we all know who we’re playing. They’re ahead of us, and we just got to worry about tomorrow.”