One of the most exciting storylines out of Spring Training this year is the battle for the fifth spot in the rotation. Roenis Elias came out of nowhere last spring and was able to post a 4.03 FIP and accumulated 1.4 WAR in 163 innings in 2014. Challenging Elias' spot is the young phenom, Taijuan Walker. While I expect both of them to see time at the major league level this year, this spring will go a long way towards determining which one of them will have to spend the first few months of the season in Tacoma.

The twist to this battle is the adjustment Taijaun Walker made to his repertoire. He has done away with the cutter that he had been using and is trying out a new slider instead. Here's what Ryan Divish had to say about it before Cactus League play began:

One thing that will be interesting to watch is the addition of a slider to his repertoire. Walker said he scrapped the cut fastball that he'd been using on occasion in past seasons for the slider to go with the slower curveball he throws. The similarities in throwing the cutter and slider pitches should make the transition somewhat easier. Walker likes the action on the slider compared to the cutter.

And here's what Divish wrote after Walker's first Cactus League start, including a quote from Taijuan himself:

Walker's slider — a new pitch he's working on — was much more effective. "I threw three, and all three of them were swing and misses," he said. The slider essentially is a variation of the cut fastball that he had thrown the past few seasons. It just breaks more, and with added depth. The effectiveness of the pitch Monday had Walker feeling optimistic. He had been struggling to find the right feel.

Here, we have a case of a pitcher making a distinct change to their arsenal. A cutter and a slider aren't really that different—the movement is generally the same and the grips are similar—but if Walker thinks he can be more effective throwing a slider rather than a cutter, then it would be prudent to investigate.

I'll be using the smallest of small sample sizes in this exercise. Walker threw four sliders total in his first Spring Training start (in the interview above, he says he threw three, but according to PITCHf/x and the video, it's really four). That's not much to work off of but it's a start.

The PITCHf/x data was updated after removing mis-calibrated data from the Charity Game on March 4.

Here's a comparison chart between Walker's cutter and his new slider:

Pitch No. Thrown Frequency Velocity pfx H Mov pfx V Mov Cutter 128 11.7% 90.35 1.87 7.26 Slider 4 11.8% 88.13 -0.53 6.29

A few things stand out initially. Walker's slider is thrown at a lower velocity, further differentiating it from his fastball. It drops vertically by almost exactly an inch more. The difference in horizontal movement is interesting and slightly confusing. A negative horizontal movement indicates that the pitch breaks in, towards a right-handed batter. Walker's new slider is breaking in, towards a right handed batter by almost two inches more than his old cutter.

That sounds pretty nasty but what does that actually look like?

Here's an example cutter from his last start in 2014, in Toronto:

That pitch looks remarkably similar to a straight fastball, thrown at a slower velocity than Walker's regular four-seam fastball. The amount of movement Walker gets on his four-seam fastball differentiates this pitch from his fastball enough that it was able to fool Bautista. But if I was looking at this GIF without any context, I'd say that pitch looks like a pretty straight fastball with the tiniest bit of break away from a right-handed batter.

Here's an example slider from his last start in 2015, in Peoria, against the Indians:

That pitch looks much different and much more deadly. It's even further away from his fastball making it a true offspeed, breaking pitch and it looks like righties will have nightmares trying to make contact with it. In fact, of the four sliders he threw in this game, all four were strikes, two of them were whiffs and one was a foul tip.

A few weeks ago, over on FanGraphs, Jeff Sullivan dove into individual pitch comparisons. He was looking at individual pitch characteristics and finding other pitchers who threw similar pitches. He considered velocity, horizontal movement, and vertical movement, and then summed z-scores of each component to come up with a comparison score.

I queried Baseball Prospectus' PITCHf/x leaderboards for all right-handed pitchers from 2007-2014 (the PITCHf/x era) who threw at least 50 sliders or cutters. I calculated comparison scores from the resulting pool of 467 pitchers and came up with Taijuan's slider's top comps.

Note: Since the PITCHf/x data was adjusted above, the comps I came up with were also adjusted. Originally, I had Josh Collmenter, Jeremy Hellickson, Michael Bolsinger, Aaron Harang, and Jake Odorizzi as top comps for Taijuan's new slider. The adjusted comps are in the table below.

Player Team Pitch No. Thrown Frequency Velo H Mov V Mov Comp Taijuan Walker SEA SL 4 11.8% 88.1 -0.5 6.3 Kris Medlen KCR CT 122 1.6% 87.4 0.1 5.7 0.8 Ryan Vogelsong SFG CT 1779 16.7% 88.3 -0.3 7.1 0.9 Alfredo Aceves NYY CT 1106 18.5% 89.8 0.0 6.5 0.9 Phil Hughes MIN CT 1576 9.9% 89.1 0.8 6.5 1.3 Ryan Dempster BOS CT 525 2.7% 88.0 -1.1 5.8 1.3

Walker's slider looks and is thrown like a cutter (this makes sense considering what he was throwing). Medlen only started throwing his cutter in 2013 right before he was lost to Tommy John surgery. Vogelsong has been throwing his cutter for much longer and it's been a decent pitch for him. Phil Hughes' cutter was a major part of his arsenal last year after dabbling with it on and off in years past.

Remember, these comparisons are only comparing pitch characteristics, not overall pitch quality. To judge a pitch's overall quality, we'd have to account for release point, deception, sequencing, and a number of other factors. That's not something we're able to do yet with the amount of data available to us.

At first glance, it seems like Taijuan Walker's new slider is a better pitch than the cutter he threw in years past. If he's going to be successful at the major league level, he'll need to have quality secondary pitches that he can call on confidently with effectiveness. Continuing to develop his slider into a weapon to compliment his plus fastball and his plus split/change means he has a greater chance of becoming a major contributor to the Mariners' future. This is definitely something to keep an eye on during the rest of Spring Training.