The Best Fastballs in the Diamondbacks Farm System

Which prospects light up the radar gun the most when they take the mound?

As continued coverage of the Diamondbacks farm system, here at Inside the Diamondbacks we are going to do a tool-by-tool analysis and pick out the best prospects for each tool. Each tool is graded on the 20-80 scouting scale, where 50 is the major league average.

Now we will be moving on to pitchers, who are much more difficult to project than hitters. While most hitting stats could be easily quantifiable based off tools, there are a lot more complexities that can make or break a pitcher's career. We'll start highlighting some of the best individual pitches in the D-backs system. The first pitch up is a pitch that every pitcher utilizes, a fastball.

Justin Martinez (70)

Full Scouting Report

The first thing that stands out about Martinez is his velocity, with a four-seam fastball that averages 100.5 MPH. The pitch, according to Statcast, has 14.5 inches of induced vertical break and 11.1 inches of horizontal movement. The pitch, compared to the typical four-seamer has more of a horizontal movement profile, but when it's coming in at 102-103 MPH the movement profile is less worrisome than the velocity. The pitch earns a 70 grade due to command issues, based off his fastball heatmap being the hottest outside the strike zone. Fastball command will make or break the young flamethrower's career, which has the potential to be either a closer or a high leverage arm in the back of the D-backs bullpen. The hope is being nearly two years removed from Tommy John surgery and a full season with his improved velocity will help him.

Yilber Diaz (60)

Full Scouting Report

Diaz is another flamethrower in the D-backs system, with a fastball that operates in the 94-98 MPH range while occasionally touching triple digits. 2024 will be an important year for him, as he'll be eligible for the Rule 5 Draft next winter and he'll start the season with Double-A Amarillo. With the need to add him to the 40-man roster, he may see a role change from the rotation to the bullpen to accelerate his arrival to the big leagues. If used in shorter bursts, his average velocity should improve as he can throw more max effort pitches. In that case, Diaz's fastball grade could bump up to 70 in next year's report.

Connor Grammes (60)

Grammes doesn't rank in the Top 30, but has a nonzero chance of making the D-backs bullpen in the near future with a plus fastball tool. A former two-way player who could touch triple digits on the mound, his feel for command hasn't developed in the way the D-backs hoped once he focused solely on pitching. After the first four years in the system was filled with adversity, Grammes has slowly climbed up the ladder of the system and is one breakout year away from getting MLB action. Like Martinez, the velocity would be a 70 pitch but loses a grade due to command issues.

Sam Knowlton (60) 

Knowlton was selected more as a stuff over command pitcher as part of the D-backs binge of drafting power arms on the third day of the 2023 draft. He ranked 2nd in all of college baseball in Stuff+ so any resemblance of command makes him a helium prospect with a big league future as a reliever. At 6'8" he creates a sharp downward angle on his high velocity fastball, he just needs one usable secondary pitch to carve out a career. We'll have a better idea of what to expect after his first year in the system and a track record to work with.

Other Fastballs to Watch:

• Landon Sims (45+): Can Sims gets his pre-surgery velocity (94-98 MPH) back? If that's the case, he's a candidate to convert to relief to keep him healthy throughout a season and quickly move towards the back end of the D-backs bullpen
• Hayden Durke (55+): Durke was set to transfer to Rice, but did not pitch in the 2023 college season due to a PED suspension. The D-backs still took him in the 13th round and signed him to a $350K bonus ($200K counted towards the pool). Durke operates in the 96-99 MPH range.

Top 30 Diamondbacks Prospects Entering the 2024 Season