The Yankees had a season filled with pleasant surprises in 2017. Almost everything went right for the squad. The lineup hit to a best-case scenario outcome while the rotation pitched far better than anyone imagined. That combination brought the Bombers to within one game of the World Series. Players outperformed expectations and the team excelled.

No pitcher better embodies that idea than Jordan Montgomery. The rookie left-hander unexpectedly won the fifth starter job and ran with it. He tossed 155.1 innings worth of 3.88 ERA baseball. His peripherals, however, sold him short. Even with a 4.07 FIP and an exactly league average 8.34 K/9 rate, it’s hard not to love what the young southpaw did.

His season didn’t play out evenly, though. Montgomery pitched brilliantly in the first half. In many ways he was a revelation for the Yankees. After the All-Star break, however, he became more of a liability. While his underlying numbers more or less held constant, his results suffered.

Jordan Montgomery’s 2017 Splits Dates IP K/9 BB/9 HR/9 GB% HR/FB% ERA FIP Dates IP K/9 BB/9 HR/9 GB% HR/FB% ERA FIP April 12 - July 7 91.1 8.57 2.76 1.28 41.6% 11.5% 3.65 4.06 July 14 - October 1 64 8.02 3.23 1.13 39.4% 10.8% 4.22 4.08

Obviously Montgomery spent a majority of the second half shuffling between the Yankees and Triple-A Scranton/Wilkes-Barre. He became the extra arm in the wake of the Sonny Gray and Jaime Garcia trades. Most observers point to a lack of regular starts as the culprit. That’s not an entirely satisfying offer, though. In fact, there’s evidence to suggest Montgomery was a different pitcher following the All-Star break.

When examining his pitch usage in 2017, two things standout. From July onward, Montgomery began to rely heavily on his fastballs. At the same time, however, he shelved his changeup. There’s almost a perfect inverse variation relationship when it comes to the use of these pitches.

Abandoning the changeup poses a problem because it was an effective pitch. According to Brooks Baseball, Montgomery maintained a 15.5% whiff rate on it. He also held batters to a .205 average and .339 slugging percentage when pulling the string. In many ways, the changeup served him well last season.

Why would a pitcher turn away from a reliable weapon? It could stem from a concern that the pitch isn’t as strong as one thinks. Pitching analysis guru Eno Sarris entirely wrote off Montgomery’s changeup early in the season, despite its effectiveness. In particular, he was unimpressed with the pitch’s lack of movement and velocity.

That seems to be the outlier opinion, though. Scouting reports typically described the changeup as Montgomery’s best pitch. Following the 2016 season, MLB.com graded it as a 55 on the 20-80 scale. The eye test backs this up, too.

Edwin Encarnacion didn’t stand a chance against that.

It’s safe to say that Montgomery’s changeup is a strong pitch. It’s unlikely he stopped throwing it because he thought it wouldn’t fool batters. On the other hand, it is possible he lost a feel for it. With five pitches in his arsenal, none of which prove overpowering, he has to rely on touch and feel. It’s not automatic for him.

One way to extrapolate this is to look at the pitch’s movement. The best changeups have a sudden, inexplicable drop. The batter reads fastball out of the hand and swings accordingly. Then a trap door opens and the ball disappears. If a pitcher has lost the feel for his changeup, then one would expect the vertical moment of the pitch to decrease. In Montgomery’s case, the opposite happened.

Save for a slight dip from July to August, the southpaw saw a gradual increase in his changeup’s vertical movement in the second half. If he could make the pitch drop like that, it’s unlikely that he had trouble gripping it. He threw it less often, but it did exactly what it was supposed to do.

The third, and most likely, explanation is that Montgomery made a conscious decision to put the pitch on the back burner. He ramped up his fastball usage and stopped going to his best weapon. He made a concerted effort to take his heater to the next level. The dissipating changeup was just an unintended consequence. How can we know this for sure? Well, he said so himself.

“I’m trying to get more aggressive with my fastball,” Montgomery recently told The State, his hometown newspaper. “As a rookie, I was a little timid to throw it...I’m going to try and be in more of an attack mode this year.”

He may best be served by reintegrating the changeup into his repertoire, though. The pitch took him to the majors and brought him great success in the first half. Assuming the Yankees don’t add another starter between now and Opening Day, the team will have to rely on Montgomery to shore up the back of the rotation. I’m guessing the club would prefer to have first half Montgomery, as well, and that may take breaking out the changeup.