Astros Rookies Diaz and Julks

Two rookie hitters in the Astros’ lineup displayed notable production in the first game of the series in Toronto. Corey Julks played LF and hit a grand slam. Yainer Diaz was the catcher and went 4 for 5 on offense. Maybe this is an opportune time to examine both rookies. Both Diaz and Julks have been used as part time players, but their offensive efforts have been valuable as rookies on a playoff experienced team.

As we review the stats for both players, the usual cautionary note about small sample size is particularly important. Small sample size applies to most statistics recorded when only 37% of the season has been completed. But small sample size is even more applicable to rookie part time players. Their sample size is based on one-third to two-thirds of the plate appearances that a full time starter has accumulated at this point in the season.

Yainer Diaz

The young back up catcher has shown encouraging signs both behind the plate and hitting the ball. I feel like Diaz has the potential to become a special player at some point in his career. By saying that, I mean a catcher who is a plus on both offense and defense is inherently special. An average major league catcher currently provides a batting average of .236, an OPS of .687, a wRC+ of 89 and Isolated Power (ISO) of .146.

Diaz’s slash line (BA, OBP, SLG), wRC+, and ISO (80 PA so far):

.284, .300, .486, OPS: .786 wRC+ 111 ISO: .203

This isn’t that surprising. As an Astros’ prospect, Diaz’s wRC+ in Class A+, AA, and AAA was 212, 121, and 121, respectively. And the pre-season projections were similarly high on Diaz.

It’s a small sample, and we don’t know if he can maintain the offensive numbers. But the projection systems’ “Rest of Season” (ROS) stats for Yainer appear to say pretty much “yes.”

The Fangraphs Depth Chart ROS projections for Diaz:

Rest of Season: .269, .307, .445, OPS: .752 wRC+ 107 ISO .176

Statcast metrics indicate that Diaz has been hitting the ball hard. Keep in mind that exit velocity stats over a sufficient sample size can be predictive of future performance.

Per Baseball Savant

Barrel% / Exit Velocity / Hard Hit%

Diaz 14.5 / 92.3 / 47.8

MLB 6.8 / 88.4 / 36.0

Ratio to MLB: 213 / 104 / 132

The “expected” batting stats suggest that Diaz’s actual batting line reflects some bad luck—or put another way, he has hit better than the actual stats. Diaz’s x-SLG is .551 (compared to SLG of .486) and his xwOBA is .362 (compared to wOBA of .318).

But Diaz’s batting performance has some negative aspects too. He has a low walk rate (3.4%), and his plate discipline stats are below average, according to Baseball Savant. For instance, he has a swing percent of 62% (MLB Avg. 47%) and a Chase rate of 49% (MLB Avg. 28%). Hopefully, Diaz can show a little more patience in a larger sample size.

Diaz’s catcher defensive stats are subject to sample size limitations. That said, the advanced defensive stats have some good signs. According to the Fielding Bible, Diaz has +2 defensive runs saved (DRS) in only 127 innings. Most of Diaz’s defensive value comes from his runs saved for caught stealing. Baseball Savant shows Diaz in the top 5% of catcher pop time.

However, Diaz does not have good pitch framing numbers at this point, which detracts from the defensive value. DRS indicates -1 runs saved due to framing, and Savant puts Diaz in the bottom 11% for catcher pitch framing.

I would like to see Diaz receive more consistent time starting at catcher. The frequency of his starts are sporadic so far. Perhaps he could pair with 2 or 3 starting pitchers each week. Not only is this an opportunity to get his bat in the lineup, but I also view it as a way to provide training for a future role as starting catcher in 2024.

Corey Julks

Corey Julks has been an appealing story this season. The 27 year old former University of Houston player was left off the Astros’ 40 man roster over the off-season, and was available to other teams through the Rule 5 draft. But he was not drafted, and he made his case to the Astros with a good spring training. The Astros put Julks back on the 40 man roster and he started the season for the first time as a major leaguer.

Julks has a part time role as a left fielder, DH, and pinch hitter. Julks’ slash line is not exciting, but it is adequate for a back up. In addition, Julks provides good speed on the bases (top one third in sprint speed) and a good arm in the outfield (top 40% in arm strength)—both good characteristics for a player off the bench.

Julks’ slash line (BA, OBP, SLG), wRC+, and ISO (160 PA so far):

.258, .275, .406, OPS: .681 wRC+ 85 ISO: .148

Although the standard slash line is unimpressive to this point, the projection systems expect some improvement over the rest of the season (ROS). Fangraphs’ ROS Depth Chart projection:

ROS: BA, OBP, SLG, wRC+, and ISO

.242, .300, .404, OPS: .704, wRC+ 95, ISO .162

Although Julks’ standard batting line is below average, his run expectancy batting is above average and suggests that he is more productive than the standard batting stats indicate. Run expectancy is measured by RE24, which quantifies the player’s ability to increase or decrease expected runs. RE24 reflects the player’s situational production, and includes the impact of productive outs and avoidance of double plays, as well as hits and walks.

Based on RE24, Julks provides above average production (+.26 runs), which is approximately 4 runs higher than his standard batting line would suggest. Julks’ 22 runs batted in (RBI) is sixth on the team, despite his limited playing time.

Julks’ walk rate and strike out rate are a cause for concern. His strike out rate of 28% is the highest among Astros’ players currently on the ML roster. His walk rate is at the bottom 1% of batters. Julks’ minor league stats are not consistent with the extremely low walk rate. That is probably why Fangraphs’ projection expects Julks’ walk rate to increase from 2.5% to 6.9% over the remainder of the season. Despite the poor strike out to walk ratio, Jukes’ plate discipline stats are roughly average and significantly better than Savant’s measurement of Yainer Diaz’s plate discipline.

Jukes’ defense in left field has been better than expected. Both Fielding Bible’s DRS and Savant’s Outs Above Average (OAA) indicate that Julks’ defense has been slightly above average. DRS and OAA both find that Julks has been 1 run above average in left field. Although Jukes’ defense is not as good as Meyers’ and McCormick’s fielding, Julks has not been a detriment on defense.

It is unclear what the Astros will do with Julks when Michael Brantley returns from the injured list. But we have no idea if or when that will occur. Until then, Dusty Baker has been rotating outfield playing time among Julks, Meyers, and McCormick. Meyers and McCormick probably provide a superior combination of defense and offense over Julks. But if you believe that competition in the outfield benefits everybody, limited playing time for Julks in left field may be helpful to the team. Julks is best viewed as a role player who can play corner outfield positions on occasion. In addition, Julks can back up 3d base if necessary.