Tuesday was a good day for the Cubs. Most of them are good days for the Cubs, but on Tuesday, they earned their first win of 2017, edging out the Cardinals 2-1. Jason Heyward and Javier Baez drove in the runs, but the star was Jake Arrieta, who spun six innings with just an unearned run going against him. He piled up six whiffs, and it was a good way for him to get things going. If the Cubs have any potential vulnerability, it has to do with rotation depth. As long as Arrieta and the other guys are clicking, the Cubs are at least as good as anyone else.

On TV, Arrieta looked effective. In the box score, Arrieta looks effective. But what was the deal with his fastball? This is an open question. I don’t have an answer. I just have evidence that makes me think thoughts.

Here is one Jake Arrieta fastball.

91 miles per hour. Harder than I can throw! Here is another Jake Arrieta fastball.

90 miles per hour. Still harder than I can throw! But I’m no bar for a major-league pitcher. Arrieta should be compared against himself. That’s where the potential problem lies.

Things are kind of foggy right now. Velocity readings are being taken differently, and that’s causing some issues, since we’d all like to make sure we’re comparing apples to apples. The system that was spitting out 2016 velocities has been replaced by a different system, which tracks velocity as best as it can right out of the hand. As a consequence, we’ve seen a good number of pitchers in the very early going who appear to be throwing harder than they used to. That needs to be adjusted for. Just most generally, though, we don’t know what to make of 2017 velocities yet. We must all proceed with a certain amount of caution, if we’re going to dive into the numbers.

Just, let me show you a big table of numbers. This is a table of average fastball velocities. These are marks for all the starters so far in 2017, and you also see where they were in 2016. Jake Arrieta stands out, and not in a way anyone would like to.

The most important first point: I don’t know what to make of this yet. And, for all these guys, we’re talking about literally one 2017 baseball game. The present situation is complicated. But if there’s a normal error, it’s that 2017 velocities are a little too high. This group, on average, is throwing harder by 0.4mph. The median is +0.7mph. That would seem to reflect the different way velocity is being recorded. Arrieta is way off. This says he’s down 2.6 ticks. Compared to the average trend, he’s down more like 3.0 ticks. It’s too early for any measure of panic, but it’s not too early to wonder what’s happening.

Looking at the Tuesday Gameday window, the tracking system seems to have broken for a short amount of time. That’s suspicious! Arrieta, therefore, has some missing pitches. So does counterpart Adam Wainwright. That’s abnormal, so it can’t be ignored. On the other hand, look at the table — Wainwright is close to the top, at +1.6mph. In the same game where Arrieta’s reported velocities were down, Wainwright’s were up. And just for the record, this isn’t only about the fastballs. Fastballs just work as a proxy. All the pitch-type velocities changed.

Arrieta last night had 58 pitches tracked, according to Baseball Savant. The fastest was recorded at 92.6, and there were three pitches that cleared 92.0. Compare that to Arrieta’s first start from 2016, when he had 89 pitches tracked. The fastest was recorded at 96.0, and there were 51 pitches that cleared 92.0. I don’t know how many more ways to show you something looks weird. I can’t explain it, but I’d like an explanation.

Losing velocity isn’t good. We just went over this the other day, in discussing Zack Greinke. It doesn’t mean a pitcher is toast, but it reduces the margin of error, and it can certainly hint at something being mechanically or structurally wrong. And if you choose to dig into Arrieta’s Tuesday start a little deeper, you can see he threw fewer than half his first pitches for strikes. He threw barely 60% of his pitches for strikes, overall. Jake Arrieta ran into real command issues over the course of 2016, which somewhat lowered his stock. This is his contract season, and he’s looking to make a big splash.

I’d love to have an answer here. I’d like nothing more than to be able to tell you everything’s okay, or everything isn’t. In reality, we’ve had one start, and velocity readings have gotten all weird, and during this very Arrieta appearance, the tracking system temporarily stopped tracking things. I wouldn’t yet advise having any sort of strong reaction. Just, you should have *a* reaction. One game in, we’ve got some Jake Arrieta-related questions, which means one could question how strong the Cubs really are. Don’t take things too far, but at least be aware of the issue. If it’s real, it could be significant.