(John McDonnell/The Washington Post)

It’s Sept. 5, 139 games into the season, and Ryan Zimmerman has 17 homers and 43 extra-base hits in 535 plate appearances. Last season, in 641 plate appearances, he produced 25 home runs and 62 extra-base hits. Over the past seven full seasons, he has averaged 22 homer runs and 58 extra-base hits over 607 plate appearances and 139 games a season. At his current rate, and without a hot final month, Zimmerman will fall short of his career averages.

The Nationals’ offense underperformed through the first half of the season and has only begun turning things around lately. Zimmerman’s struggles are part of the reason. He is hitting .277/.351/.443 in 125 games — all respectable numbers — but underwhelming for such an important piece of the Nationals lineup. The $100 million third baseman’s .795 OPS ranks 56th among qualified hitters. Among third basemen, his OPS would rank eighth in the majors, behind Seattle’s Kyle Seager and ahead of Baltimore’s Manny Machado.

The most disappointing part of Zimmerman’s hitting has been his lack of power. His .443 slugging percentage is more than 30 points lower than his career average. His isolated power (.166 ISO) is down from his career mark (.189). He has been hitting the ball hard (27 percent line drive percentage is higher than career 21 percent rate) and he hasn’t been unlucky (.330 BABIP compared to .319 career BABIP). The home runs have just been less frequent. While he hit .292 in July, for example, Zimmerman produced only three homers. In August, he hit .264 with only three homers. But in four games in September, he has already smacked two long balls.

“It’s starting to get a little better,” he said. “I’ve felt pretty good all year. For some reason, I haven’t got … if I can get hot here in the last couple weeks, I’ll be right where I usually am, anyway. It’s one of those things, I wish I knew the answer. It’s just been one of those years.”

Over the final month, the Nationals would benefit from a Zimmerman hot streak. This season, they are 28-11 in games in which he notches an extra-base hit, including 11-4 when he homers.

In order for Zimmerman to match his career averages, he would need a scorching September. Over his career (full season from 2006 to 2012), he has homered once every 28 plate appearances. This season, he has homered once every 31 plate appearances. In order to match his career averages, he would have to homer once every 14 plate appearances over the final 23 games.