Along with Jeremy Hazelbacker, Aledmys Diaz's unsuspected success has been a big part of the reason why the Cardinals' offense has done as well as it has.
With Ruben Tejada back on the Cardinals' 25-man roster (and Greg Garcia heading back to Memphis), the Cardinals will have a tough decision to make: Who plays shortstop? After Jhonny Peralta got hurt in Spring Training, Tejada was brought in as the answer to this question. Jedd Gyorko was never really a real shortstop, Greg Garcia might not be much more than a nice organizational piece as a AAAA talent, and Diaz, while interesting, could use some more seasoning at Triple-A, where he's only had 58 career plate appearances thus far.
Of course, then Tejada got hurt, and the Cardinals were back in the same situation. Gyorko originally was given the most chances, getting four of the first six starts at shortstop (and one of the two games he didn't he played second) but Diaz has made the most of the playing time he's been given, running over a 200 wRC+ so far on the year. As a result of Matheny's dictum to play the hot hand, Diaz has started the last 6 of the last 7 games of the season, all at shortstop. For the moment, he has seized the upper hand over Gyorko and Garcia, both at the plate and in the field.
One of Matheny's most consistent traits has been to play the hot hand. Although a lot of us here try not to give small sample sizes too much weight, it's not a bad thing by any means to reward the players who are doing well with more playing time. Players aren't statisticians, and it would be hard to take a demotion after playing so well. Any attempt made by the org to say "Well, you did perform well, we just don't think it's sustainable" would probably not be received well. It's the same reason why Hazelbaker is going to keep getting in the everyday lineup for now. As long as he's producing, why take him out? You just have to hope that, once he cools off, Matheny moves on to trying to find another hot hand.
With Diaz though, there are some very encouraging signs that this hot streak has some realness to it.. No, he's not going to keep his wRC+ in the 200's, but some underlying statistics are very encouraging. First, let's look at his overall hitting profile:
Low walk and strikeout rates, super high BABIP and ISO rates. the BABIP and ISO will inevitably cool off, but the question is, to what extent? Those super low walk and strikeout rates are very strange, even in a short sample size, but check out the plate discipline stats:
*Note that while the overall numbers include last night, these stats do not.*
Any idea that Diaz's walk and strikeout rates are due to him being very aggressive should be thrown out the window. He's actually swinging at significantly less pitches out of the zone than average, while swinging at a bit higher than average rate of pitches in the zone. He's getting pitches in the zone a more often than average, which seems to be typical pitcher behavior towards rookies who weren't super-celebrated prospects. That is definitely something that pitchers will adjust to, and I would expect his Zone% to move towards the league average as long as he keeps getting more PA.
Diaz's strike zone management is encouraging, but let's look at some of his batted ball stats:
*same as the plate discipline numbers, these do not include last night's plate appearances.*
A lot to like so far. He's in the top 10% in Hard Hit%, 21st out of 226 players with over 30 PA. That goes along with above average Line Drive% (53rd, putting him in the top 25%).. It gets better: Among 193 players with more than 20 tracked batted balls by Baseball Savant, Diaz is 9th in average Exit Velocity (top 5% in baseball), at 95 MPH. He places 23rd in average distance, at 247 feet.
Diaz's start is unsustainable, in the sense that he's not going to be able to keep up that 200 wRC+. However, he has been hitting the hell out of the ball, both according to the eye test and the advanced stats. I don't expect those to continue at the same rate either, as he's probably not one of the best players in the game in terms of contact quality. But he's doing it right now and that has to count for something. This isn't Pete Kozma in September and October of 2012 getting weak contact to find holes over and over, this is the result of a guy who makes a lot of great contact, running hot and also having some things go his way. Both things are unsustainable, but one is going to be due for a much larger fall-off.
Even if Diaz may settle in around the middle of the pack in terms of hitting, that's still a great place to be for someone who can play shortstop, as the position on average produced a 85 wRC+ in 2015. Even if he plays a manageable below average defense at shortstop (say, five runs worse than average), if he can and maintain a 110 wRC+, that's nearly a 3 WAR player with five more years of control after this season (six if he returns to the minors for even a fairly short amount of time). The projections don't quite believe that yet, as he holds an average projected wRC+ of 92 from Steamer and Zips, but that's up from 85 to start the year. Projections are conservative, and for just two weeks performance that's actually a pretty big jump in expectations. For players with little MLB service time there's also very high error bars, doubly so for someone like Diaz with a lack of minor league sample size as well.
One thing that is a little concerning is the really high Pull rate. It's a small sample of course, but if those don't change he will be pretty easy to shift against. If it is a real issue and not just short sample size, Major league pitching will expose it. I don't think it's something to worry about at this stage though.
The advantage of bringing in Tejada was raising the floor for the Cardinals at shortstop. When the Mets first placed Tejada on waivers I voiced my opinion that I'd rather just let the Cardinals try out their internal options, but even I felt a little more comfortable once he was a Cardinal, despite noticing several shortcomings in his profile. Diaz has the higher ceiling at this point in their respective careers, but going into the year with such a low floor at shortstop was too risky, especially after a disastrous year from the position in 2013 still alive in many people's memories. Even in this ultimate short sample size theatre though, they have to be feeling much better about their non-Peralta options, and that should affect Tejada's playing time going forward. The Cardinals shouldn't simply dump Tejada, but he shouldn't be considered the presumptive starter in Peralta's absence either. Diaz has earned that title for now.
One thing I've noticed in the comments is some calling for Diaz to stay at short when Peralta returns, with Peralta moving to third and Matt Carpenter moving to first. This would be great for the infield defense. While i have been high on Peralta's defense, he did seem to lose a step last year, and that might be the step that makes playing shortstop untenable. It's too early for me to know how good Diaz really is at short, but besides booting a few, he has shown the capability of being just fine out there. Peralta would be a huge upgrade over Carpenter at third. I like to think of Juan Uribe as a good comp for an aging Peralta: strong middle infield defender who, once losing the range to play shortstop, moved over to third where his defense was good enough to make him quite a valuable player despite only hitting a little above average.
Likewise, the Cardinals first base defense would be improved as well, from the shoddy defense of outfielders turned first basemen in Brandon Moss and Matt Holliday, to that of a below average third basemen in Carpenter. The positional adjustment from the average third basemen to the average first basemen is 15 runs, so while Carpenter is a below average third basemen, he'd be expected to be one of the best fielding first basemen in the league. Everyone reacts to a position change differently, but as a career infielder, I would bet on Carpenter making a better transition to first than Holliday, Moss, and Stephen Piscotty has.
That's too far away to really be thinking about too much right now, but it's a more realistic option than what we thought just a couple weeks ago. For now, we can stay focused on whether he's currently a better option than Tejada. As far as that goes, I don't think I'm alone in saying "Yes" to that question.