Patrick McDermott / AP

The most obvious explanation for Jordan Zimmermann’s recently flagging performance is fatigue. Once you look deeper, it is also the wrong explanation.

After he allowed the Cardinals a career-high eight earned runs in 3 2/3 innings Saturday, Zimmermann reached 164 2/3 innings on the season. Because the Nationals shut him down at 161 2/3 innings last year, he has never carried a workload like he’s had this season. Zimmermann had a 2.28 ERA in his first 21 starts and a 6.23 ERA in his last six, and so Zimmermann wearing down is a reasonable conclusion.

But his pitches suggest the opposite: Zimmermann may have been too strong on Saturday. This year, he has averaged 93.8 mph with his fastball. Saturday, he threw it 94.3 mph and touched 96.2. Zimmermann pitched on an extra day of rest Saturday, which he said often spells trouble.

“That’s probably why the velocity was up, and my control was terrible,” Zimmermann said. “I just got to focus on not overthrowing and stay within myself.

“I think I pitch a lot better when I’m on regular rest. My arm doesn’t have an extra day to recover and feel that much better. I don’t know if this is the right way of saying it – I pitch better when my arm is tired. Five days is perfect. Six, you got the extra day of rest, you feel that much stronger and loose and everything else.”

The problem of more power than usual mattered even more when he threw his slider, his best out pitch.

Zimmermann has averaged 86.4 mph with his slider this season. Saturday, he averaged 87.7 mph, and one slider zipped at 90 mph. Think about in golf, when you strike a putt too hard and the ball rolls through the break of the green rather than riding it and curving toward the hole. Zimmermann was throwing his slider through the break. Essentially, he turned a wicked slider into a so-so cutter.

“It’s definitely too hard,” Zimmermann said. “I don’t want it too hard. It doesn’t move as much, and it’s that much closer to my fastball. That’s when I get myself in trouble. I need a little differential there.”

For several starts in July, Zimmermann had trouble loosening up before starts. It would take him 10 or 15 tosses before he could get the feeling of inflammation out of his shoulder before a start. “Then he got nice and loose and he was fine,” Manager Davey Johnson said. “I was really concerned then, but through all that he pitched great.”

Zimmermann said his shoulder is no longer an issue. Both he and Johnson agreed his issue Saturday was mechanical, not physical. In his delivery, he would open his front shoulder too early, which caused the ball to stay up and over the plate.  

“When you open up and jump at the hitter, the elbow drops and the slider is flat,” Johnson said. “That’s what he was doing against the Cardinals.”

This afternoon, Zimmermann threw his usual bullpen session and focused on fixing the flaw.  “I really felt good, and I got it straightened out,” Zimmermann said.” Everything went well.”

Zimmermann will make his next start Thursday against the Cubs. It will be another step into uncharted territory, approaching 170 innings. But he insists he feels great, and with playoff baseball a distinct possibility, he does not think that will change.

“I don’t see myself tiring anytime soon,” Zimmermann said. “So that’s a good thing.”