No one should have expected significant improvement in the Washington Nationals’ offense this season.

Pitching and defense is the foundation of General Manager Mike Rizzo’s long-term plan, so he focused on upgrading the ballclub’s fielding. And if the Nationals’ top pitching prospects develop as Rizzo expects, they could benefit from working with a lineup of sure-handed position players. Run production is still important, too, though that’s often hard to tell watching the Nationals.

The team has taken a big step backward at the plate, minimizing its progress in the field. Rizzo did little to address the Nationals’ glaring deficiencies in the Nos. 1 and 2 spots in the batting order — the positions that fuel an offense — and other shortcomings are obvious. Rizzo assembled a roster that lacks table-setters, has limited flexibility and little power, leaving Manager Jim Riggleman to deal with the mess.

Of Major League Baseball’s 30 clubs, only San Diego has a team batting average lower than Washington’s .224. The Nationals also have a National League-worst .295 on-base percentage.

Although the relative importance of basic batting average is debatable, old-school ball guys as well as the sabermetrics crowd generally agree on the value of on-base percentage. Base runners are a fundamental component of run production. The Nationals’ inability to consistently get on base — especially in the top two spots — has derailed their offense for almost a quarter of the season.

In addition, the Nationals also strike out too frequently for a team that doesn’t hit many home runs. Combined with their poor on-base percentage, that’s a formula for offensive ineptitude.

For the most part, the Nationals have pitched well. They have been effective in the field, and often outstanding. Their success in those areas has helped the Nationals (16-18) remain around .500 despite being outscored by 19 runs.

Obviously, the loss of third baseman Ryan Zimmerman has been a major blow to the offense because “you’re talking about an impact player, a real game-changer,” veteran infielder Alex Cora said. “Because of everything he does, you’re talking about a guy you really can’t replace. But nobody is going to feel sorry for you.

“You can’t use it for an excuse. We know we haven’t even been decent offensively as a team yet. We know our pitching is the reason we’re even at where we are right now. As simple as it sounds, we’ve just got to swing the bats better.”

It appears doubtful, however, that the Nationals’ batting order possesses the potential to make substantial strides in season.

The Nationals expect Zimmerman to fully recover from abdominal surgery and return to the lineup sometime in June, “and when we get him back, it’ll be like making a big deal at the trading deadline,” Riggleman said. “When you can add someone who impacts your lineman like he does . . . it’s like a big addition you weren’t counting on.”

Zimmerman has played in only eight games. Even if he returns sooner than expected, no one knows how long it could take him to regain his timing. And getting a boost from Zimmerman in the middle of the batting order doesn’t solve the Nationals’ problems at the top or off the bench.

Then there’s Jayson Werth. Not surprisingly, the team’s new right fielder is off to a slow start, struggling under the weight of the enormous expectations that accompany a $126 million contract.

In 139 at-bats, Werth has only four homers and nine runs batted in. He’s batting .227 with a .324 on-base percentage and an OPS (on-base plus slugging) of .710 — by far Werth’s worst statistics in each category since he became an established everyday player with Philadelphia in 2008. Although Werth can’t drive in runs if no one is on base, he hasn’t been a productive hitter.

I covered Werth early in his career when he was with the Los Angeles Dodgers. He worked hard and displayed the right attitude, so I wasn’t surprised he became a good major leaguer.

I was stunned that Rizzo gave Werth such a big contract that included a complete no-trade clause. In terms of position players, those types of offers are usually reserved for the game’s elite power hitters.

Werth has hit more than 30 homers only once in his career. He has never driven in at least 100 runs.

He is considered a premier defensive player, which Rizzo clearly values, and Werth has been a big part of the Nationals’ defensive improvement. But the club probably should expect only about 25 homers and 80 RBI from Werth.

Rizzo declined to re-sign slugger Adam Dunn, who averaged 38 homers, 104 RBI and 96.5 walks during two seasons in Washington. Dunn is a defensive liability, so he didn’t fit the blueprint.

Owner Ted Lerner is all in on Rizzo’s vision for the franchise, and Rizzo is confident he’s right. It seems the Nationals will either succeed or fail with an emphasis on defense, but Rizzo should spend at least a little time working on the offense in the future. Especially if he’s at all interested in something called balance.