The first thing to know about Julio Urías is the first thing that everyone already knows about him: He's really, really young. Historically so, actually. When he takes the mound for the Dodgers in Game 4 of the NLCS on Wednesday night, he'll become the youngest postseason starter in Major League history. The Dodgers lead the series, 2-1, after posting a 6-0 victory over the Cubs in Game 3 on Tuesday.
The Dodgers' championship drought was already nearly eight years old when he was born. Vin Scully was nearly five decades into his broadcasting career when Urias was born. We could do this all day. He's young. But while you'll hear a ton about his age, "20" isn't really the most important number for Urias.
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What's important is the type of pitcher Urias has proven himself to be in his rookie year, regardless of what his birth certificate says, because he's still evolving -- especially, perhaps, with his curveball. For example: If you look at the totality of Urias' first regular season, you'll see some pretty solid numbers.
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Urias had a 3.39 ERA and a 3.17 Fielding Independent Pitching mark, both above-average and more or less the same as what Texas ace Yu Darvish posted this year. That alone makes it one of the most impressive seasons by a teenager in modern history. (Though he turned 20 in August, it's still considered his "age-19 season").
But even that high praise may be underselling him a bit. Urias' first two appearances (eight earned runs in 7 2/3 innings) didn't exactly get him off to a good start, and did a fair bit to impact his overall season line. Since then, he's thrown 69 1/3 regular-season innings with a 2.73 ERA and a 2.60 FIP, and those numbers are spectacular. After that brief acclimation period, Urias basically pitched like Kelvin Herrera (72 innings, 2.75 ERA, 2.47 FIP), and he did it from the left side.
Now, it's fair to note that one of those two poor initial starts came against these same Cubs, when he allowed home runs to Kris Bryant, Jason Heyward and Javier Báez in a 7-2 loss in Chicago on June 2. But that also accounts for 60 percent of his season home run total, as he's allowed only two since, and he also tied a career high with eight strikeouts against the Cubs in a 3-2 Dodgers win on July 27.
In comments before NLCS Game 3 on Tuesday, he seemed to believe that the familiarity edge resided with him, not the hitters.
"I've seen them twice, and that really helps. It makes me a lot more comfortable, especially being that the second time was here at home," he said.
Even if the Cubs feel the same way, the truth is that this isn't the same Urias that they saw back in June, and a big part of that is in how his secondary pitches are deployed. Then, as now, Urias throws his quality four-seam fastball slightly more than half the time, and it's easy to see why. Among lefties who have thrown 200, it's got top-8 spin, top-6 movement, and top-7 whiff rate. It's a good pitch, and it's why he relies on it. It's the way he uses the supporting pitches that have changed, however.
"He's just very methodical," said Dodgers bullpen catcher Steve Cilladi. "You can tell he's always thinking."
Nowhere is that more evident than in the change we've seen in his curveball as the season has gone on. Urias used to have a sweeping, horizontal curveball, the kind that's often tantalizing for right-handed hitters when it comes in from a lefty pitcher. It was, in fact, one of the oddest lefty curves in the game, in that after his first four appearances, it had an average of 8.2 inches of horizontal movement and very little vertical break at all.

The average Major League curve has 3.5 inches of horizontal movement and drops 5.2 inches, which is more like you'd expect a curve to be. It's not quite like that any longer. The movement on his curveball changed throughout the summer, and Urias confirmed through his interpreter that he had changed his grip on the pitch, crediting Dodgers pitching coach Rick Honeycutt for helping him find something that works.
"It's a pitch that I've been working on since I was in the Minor Leagues, and thanks to Honeycutt, I was able to perfect it," Urias said. "Now, it's one that I feel very comfortable and I'm confident that I can get outs with."
Let's show you the difference, in a pair of visuals. The first one is the GIF below, which has two different Urias curveballs overlaid on top of one another. Though they're just two months apart, you'll see they look extremely different. The ball you see coming in high was thrown against the Rockies on June 7, in his third start, sweeping across the zone with 11.1 inches of horizontal movement and just under one inch of drop.
The lower one was thrown to Philadelphia's Odúbel Herrera on Aug. 8, and it cut that down to just two inches of horizontal movement, moving less side-to-side and getting lower in the zone.
Gif: Urias' curveball adjustment
Since Aug. 1, Urias threw 86 curves, had 12 put in play, and saw just two reach 100 mph of exit velocity (one of which was a groundout), with an average exit velo of 85.1 mph. Prior to that, he had 16 put in play, and four were triple digits, averaging 90.3 mph. It's not suddenly his best pitch, it's just the continued evolution of a young pitcher finding his way. We can also compare the horizontal movement on Urias' curve to that of his slider. Where they were once extremely different, now they move horizontally in a very similar way:

Sometimes, being too similar is a problem, but it can also help fool the hitter into being unsure which pitch is which -- and since Urias has added extra drop on his curve, getting it on a lower plane than the slider, the deception out of his hand may be increased. It's not that this is suddenly The One Weird Trick that is going to allow Urias to dominate Chicago, though it's worth noting that no team in baseball had a lower exit velocity on curves than the Cubs, who hit a below-average .211 against them. It's just that a young pitcher is always changing, always trying to improve.
"More than anything, he's feeling out what it's like to throw it properly," Cilladi added. It's maybe not quite there yet. It is evolving, though, and it's not what the Cubs may have seen earlier. It's just another thing for them to watch out for in Game 4.