Can Jake Arrieta recapture the magic? What to watch for on the mound

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Admittedly, that's laying it on a bit thick. But that's what numbers like Arrieta put up can do to a sports writer. From June 4, 2015 until May 16 of this season, Arrieta went 27-2 over 33 starts with a 1.36 ERA. It was real in that it happened, and it was unreal in that it could not keep going.

Since then, Arrieta has been a solid major league pitcher, going 8-7 with a 3.68 ERA. That's good. It's fine. The ERA is about 5 percent better than the big league average over that span. It ranks 32nd among 79 qualifying pitchers. Meanwhile, teammates Jon Lester and Kyle Hendricks have left Arrieta behind in pursuit of the Cy Young Award Arrieta won last season.

Arrieta takes the mound Friday against the St. Louis Cardinals. Mostly likely, he'll have one more regular-season start after that but this is probably the last one played under true competitive circumstances. Manager Joe Maddon has pledged to play this weekend series straight up out of deference for the Cardinals' place in the wild-card derby. After that, the Cubs' player usage patterns will be more spring training-like. On top of all that, the afternoon tilt will be nationally televised on ESPN.

Suffice to say, there will be a lot of scrutiny on Arrieta's performance. By highlighting the differences between Arrieta the Great, and Arrieta the Good, maybe we can guide some of that scrutiny. Here are three areas to focus on.

Fastball velocity

There was a lot made about Arrieta's velocity after his last start on Saturday. His average fastball speed this season is down to 93.7 mph this season, 0.9 mph less than a year ago. Since the beginning of August, it has been down to 93.2. In three starts this month, it has been at 93.0, as it was Saturday. He touches 96 mph but his working range has been more 92 to 95 than 93 to 96. What may be important to keep in mind is that the average velocity this season is more in line with Arrieta's career norms. It's last season that was the aberration.

Arrieta says that he has been working on some things in an effort to conserve energy for the later innings. Perhaps. But his results haven't been good enough to get him into the later innings. That may have nothing at all to do with fastball velocity. His average break length on the hard stuff (4.9 inches) is a career high. He's getting more movement. And the results have been fine: Arrieta's OPS allowed on fastballs (.587) is roughly the same as it was last season (.580). Since May 31, however, it has been .633.

Fastball command

On Saturday, Arrieta seemed more perturbed about the location of his fastball than the velocity. The theory seems sound. If he gets ahead in the count, hitters have to be more aggressive with pitches near, but not in, the zone and his off-speed stuff then becomes that much more dangerous. It was a pattern of attack we saw him execute again and again and again when things were going so well. As we pointed out the other day, it's a big deal. This season, batters have a .689 OPS in plate appearances when he misses with the first pitch. When he jumps ahead, that number plummets to .426.

Of course, Arrieta doesn't throw a hard sinker with the first pitch to every batter, but he has done so a lot more often this season. Last season, he started with a fastball 66.1 percent of the time. This season, that's up to 78.6 percent, by far a career high. He used to start more batters with a curve, but now that's a rarity. Last season, he threw a slider as his first pitch 21.8 percent of the time, but that's down to 12.8 this season.

Why the change? It likely has to do with that desire to get ahead in the count. He has thrown first-pitch strikes with his fastball 59.8 percent of the time this season, down 2.1 percent from last season. Since May 31, he's at 58.3. This month, he's at 55.6. Things are trending in the wrong direction.

The slider

During his hot streak, Arrieta threw sliders 26.7 percent of the time. His results were stunning: an OPS allowed of .498, a strikeout percentage of 25.7, a well-hit average of .090. When you envision Arrieta at his best, it's him putting away a batter flailing hopelessly at a slider. During the 2015 portion of the streak, the slider was the decisive pitch on 38.5 percent of Arrieta's strikeouts, 6 percent more often than his fastball.

Since then things have been a little different in slider land. The fairy dust Oberon blew in his ear must have given him a magical but fleeting slider. (Sorry, cannot let go of that analogy.) Beginning with the end of the streak, May 31, Arrieta has thrown sliders just 15.7 percent of the time. He has allowed an OPS of .722 on the pitch, and used it as the decisive pitch on just 16.5 percent of his strikeouts.

In the end, we may be talking talking a chicken and egg proposition. The problem with zeroing in on any one aspect of a pitcher's arsenal is to sidestep the fact that the tools all work together. Maybe the slider is the symptom, or maybe it's the disease. Whatever the case, if there are a flock of Cardinals flailing at Arrieta's slider on Friday, Cubs fans might get even more excited about the upcoming postseason than they already are. As if that were possible.