What else does Tanner Houck have to do?

On Saturday, Tanner Houck picked up right where he left off following his explosive (yet brief) introduction to the MLB level at the end of the 2020 season. In fact, by some metrics, besides a few defensive miscues behind him and a bit of bad luck, Houck was even better in his first start of 2021 than he was in his three starts last year overall. Although the 24-year-old right-hander had a 0.53 ERA in those three starts last year, his marks in FIP (3.25) and xFIP (3.73), while still good, were not as otherworldly. In his start on Saturday, Houck ended with an ERA of 3.60 after giving up two earned runs over five innings, but his FIP (0.87) and xFIP (2.21) were positively Pedro Martinez-esque.

The key to Houck’s success and the most exciting aspect of his game to this point, small sample sized be damned, is his slider. It’s a pitch that breaks so devastatingly that it leaves batters either swinging at air or watching as it snaps into the strike zone. Just take a look at this one from Saturday.

In that first start, Houck’s slider averaged 15.8 inches of horizontal movement, according to Baseball Savant. That would have tied for the 11th-best slider break among qualified pitchers in MLB last year. Houck actually finished 24th in the category in 2020, with an average of 13.9 inches of horizontal movement, so it’s possible this pitch, which was already great last year, has only gotten better. That is terrible news for opposing batters because Houck’s slider has been unhittable — and I mean that literally.

This tweet was from before Houck’s day was finished, but he has still yet to give up a hit on his slider, while he has 13 career strikeouts with it. He has 16 career strikeouts among his other pitches combined.

Houck’s slider hasn’t been effective because he uses it sparingly, making for very few chances for batters to figure it out. In fact, Houck threw as many sliders as fastballs on Saturday (32) and he threw his slider 35.5 percent of the time compared with a 37 percent rate for his fastball last year.

Houck’s slider is clearly MLB-ready and may just be among the best in the game already. However, despite having such a devastating offering, he is not an official member of the starting rotation for the Red Sox right now and is expected to be sent back to the alternate site. Although there’s an argument to be made that Houck has already earned a starting role at the MLB level, the folks pulling the strings aren’t convinced just yet. So let’s take a look at a few of the things Houck should work on to make it impossible to keep him out of the rotation.

We’ve already gone over how good Houck’s slider is, but he also has an excellent four-seam fastball. He nearly averaged 95 miles per hour with it on Saturday, which is up from roughly 93 miles per hour last year. Assuming he can keep living near the mid 90s with his heater while tossing a wipeout slider in the low to mid 80s, Houck already has the foundation of an exceptional repertoire. But Houck could use a few more pitches that match his effectiveness with his slider and fastball. He may already have one of them, even if it hasn’t gotten as much publicity.

Houck’s sinker is his third pitch, as he’s thrown it between 20 percent and 25 percent of the time during his brief MLB career to this point. It also has a different movement profile than his top two pitches, averaging 31.8 inches of vertical movement last year (30.2 inches on Saturday). Houck also reaches into the lower 90s with his sinker, which is why he ranked third in sinker drop among qualified pitchers vs. average last year, according to Baseball Savant. Meanwhile, his slider, which actually has even more vertical break, isn’t all the different than most other sliders on a vertical movement basis.

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Houck also features a splitter, but he throws it far less frequently (three percent of the time in 2020 and 3.5 percent of the time on Saturday). It also lacks the consistently eye-popping break of his slider and sinker. However, if he could improve on this pitch a bit more and work it into the mix more often, Houck could have a solid four-pitch mix that would make it even more difficult for hitters to zero in on what he’s offering. In particular, the splitter will likely be necessary to consistently get lefties out over the long haul. Maybe he could even mix in another offspeed pitch along the way, but just perfecting his sinker and splitter would be enough for now.

In addition to rounding out his repertoire, Houck also needs to build up his stamina. He has averaged 5.5 innings per start across four MLB appearances. That’s not too bad and some of it is out of his control. To begin with, pulling starters earlier in games is becoming the norm, and that’s without even considering how the Red Sox may want to be careful with putting too many innings on their budding ace’s arm too early in his career.

However, even if he doesn’t get the chance to throw deep into games that often, having the ability to do so would be useful all the same. Even on Saturday, Houck started to slow up a bit in the later innings. He allowed two hits and struck out six batters in the first three frames. He let up four more hits and only struck out two more batters the rest of the way. He also gave up his only walk of the game in that stretch. Although some of those innings were extended due to faulty defense, Houck clearly wasn’t as effective. That’s understandable, as every pitcher loses effectiveness each time through the order, but being able to fend off that regression for a bit longer could turn Houck into a certified ace rather than just a promising starter.

There’s not much else Houck needs to do to establish himself as an MLB-ready starter. He throws strikes at a roughly league average rate, has already been effective in a short stretch of games at the MLB level and is clearly headed toward a larger role in the near future. To accelerate that timeline, along with a few tweaks here and there, Houck just needs to solidify his arsenal of pitches and prove he can pitch deeper into games. If he can do that, the Red Sox would be (even more) foolish to leave him out of their big league rotation.