Last week, we examined the difficulty the since-demoted Chris Tillman had in retiring big league hitters. But he’s not the only highly-acclaimed Orioles pitching prospect whose transition to the majors has been bumpy. Nine starts into his rookie season, Jake Arrieta has walked far more batters than he has fanned. Just yesterday, the TCU product failed to strike out a single Minnesota Twins batter while issuing four free passes. Why is Arrieta pitching so poorly? Let’s try to find out.

A 6-5, 220 pound right-hander who fell to the fifth round of the 2007 draft due to bonus demands (the O’s anted up with a $1.1 million offer), Arrieta began his pro career in the High-A Carolina League in 2008. He used his 92-94 MPH fastball, mid-80′s slider, high-70′s curveball and mid-80′s change to strike out 9.6 batters per nine innings, but his control was so-so, with 4.1 BB/9. Arrieta’s park-and-league-adjusted FIP, according to Minor League Splits, was 3.42 in 113 innings pitched. Last year, he split the season between the Double-A Eastern League and Triple-A International League. Arrieta had a combined 8.8 K/9, 3.3 BB/9 and a 3.91 adjusted FIP in 150.2 innings.

Arrieta was named the 99th-best prospect in the game by Baseball America prior to the 2010 season, with BA saying his “pure stuff compares with any of the Orioles’ elite young pitchers, but his command puts him a notch behind them.” ESPN’s Keith Law ranked Arrieta 90th on his top 100, calling him “a four-pitch guy with no plus pitch but nothing below-average.” Prior to his early-June promotion, Arrieta had 7.9 K/9, 4.2 BB/9 and a 4.11 adjusted FIP in 73 IP. The good-not-great K rate and inflated walk rate portended some turbulent times in the majors — Arrieta’s major league equivalent line called for 6.3 K/9 and 5.35 BB/9 in the show.

To this point, the 24-year-old’s control of the zone has been even worse. He’s got 3.91 K/9 and 5.21 BB/9 in 48.1 IP, with an xFIP (6.10) that’s more depressing than his 5.40 ERA. As pointed out in the Tillman article, it’s important not to draw any definitive conclusions from a small sample of pitching. But right now, he’s not missing bats and he’s not locating his pitches.

Arrieta’s swinging strike rate is 5.5%, a figure which puts him in the same company as Livan Hernandez and Ryan Rowland-Smith. For comparison, the MLB average is 8.4%. He’s getting batters to expand their zones fairly often, with a 32.1 outside swing percentage (28.9% MLB average), but they’re making a ton of contact on those out-of-zone pitches. Arrieta’s outside contact rate is 80.5% (66.5% MLB average). With hitters also squaring up plenty of in-zone offerings (his 90.6% Z-Contact rate is above the 88.2% big league average), his overall contact rate is 86.4%. That’s well above the 80.9% MLB average.

According to Pitch F/X data from TexasLeaguers.com, Arrieta’s fastball is getting whiffs 4.6% of the time that he throws it (5-6% MLB average). His changeup (14.7%) has gotten more whiffs than average (12.6%), but his slider (11.1%) falls short of the 13.6% MLB average. Arrieta’s curve has been a bat magnet — it has a one percent whiff rate (11.6% MLB average).

As R.J. Anderson pointed out, Arrieta isn’t putting away hitters when he gets in 0-and-2 counts. Baseball-Reference shows that he has gotten in 0-2 counts 22% of the time, close to the 23% MLB average. In those situations, Arrieta has been bashful about breaking out his breaking stuff — he’s using his fastball 61% of the time in 0-2 counts, compared to the 49% MLB average. He has yet to record a K in an 0-2 count.

On a related note, he’s also struggling to throw strikes. Arrieta’s first pitch strike percentage is 54.3%, compared to the 58.8% MLB average. Just 43.4% of his pitches have caught part of the plate (47% MLB average). His fastball is getting strikes 63.6% (62-64% MLB average). Everything else, though, is missing often. The slider has been thrown for a strike 53.8% (63.4% MLB average), the curve 51% (58%) and the change 55.9% (60.7%).

Arrieta’s throwing his fastball for strikes, but he hasn’t gotten a feel for his slider, curve or change. That has led to lots of hitter’s counts. And, when Arrieta has backed the hitter into a corner, he’s not sealing the deal — in plate appearances reaching a two-strike count, he has gotten a K 23.9% (36.5% AL average) and he has walked the hitter 18.2% (8.3% AL average). That tends to happen when a pitcher can’t get a called strike or a swing-and-a-miss with his breaking and off-speed stuff.

Again, all of these numbers come in less than 50 innings. It’s not unusual for a pitcher to stumble during his first taste of big league action. Arrieta doesn’t look like a future front-of-the-rotation type — as Law said, none of his pitches are awe-inspiring, and BA noted that his control isn’t great — but he’s still someone to monitor in keeper leagues. As for the present moment, you might want to let Arrieta work out the kinks on somebody else’s roster.