Danny Santana had a fantastic rookie season. Santana's .319/.353/.472 slash line and his OPS+ of 130 are fantastic numbers for anyone, let alone a 23-year-old in his first big league season. Because of Santana's youth and the fact that he had what appears to be a breakout rookie season, the expectation for Santana is that he will continue to improve and build upon his great rookie season.
To be blunt: That's highly unlikely. His strikeout to walk ratio was 98 to 19, and very rarely do players who can't control the strike zone continue to hit at a high level year after year.
It's important to note that a key benefit of advanced stats is that they can help paint a very clear picture of what would be expected the following season. Things like batting average on balls in play (BABIP) are based far more on luck than skill, and generally if a player has a very high average in this stat, his numbers will fall off quite a bit the next season. For example, if someone's BABIP is between .280 and .295 over the first four seasons of their career, and then suddenly spikes to something like .354 in year five, it's very likely that this players sixth season BABIP is far more likely to be between .280 and .295 than it is to be anywhere near .354 again. A higher BABIP will boost each part of the slash line, as a .250/.300/.390 hitter could become a .275/.340/.420 hitter, becoming 10% more effective purely based on luck.
Saying that, one thing that does bother me about these kind of advanced stats is that I think people use them too often to judge the prior season incorrectly. What I mean by that is too often players who got lucky and had a big season thanks in large part to simply dumb luck aren't given the credit they deserve. Sure, the player got lucky. But the point should be to see who brought the most ACTUAL value to the team, not just perceived value. Getting lucky, while completely out of the player's control, still adds value to the team. A player who improves his offensive output by 10% because he just improved his swing or strength in the off-season is worth the same amount as a player who improves his offense 10% just by dumb luck.
Anyway, back to Danny Santana. The 23-year-old center fielder/shortstop was a good player last season. However, he was mostly a good player because he got lucky. His .319 batting average is largely due to a completely unsustainable .406 batting average on balls in play. Santana narrowly missed qualifying as he received just 430 plate appearances, but the league leader hit .373 on balls in play, while the league average was .299. Had Santana hit .299 on balls in play, he would've had 90 hits instead of 129. If we just pretend all of those hits were singles (highly unlikely, but easier for computing the slash line and kinder to Santana) that would've led to a slash line of .222/.262/.375. Suddenly Santana's rookie season doesn't look so fantastic.
But again, Santana getting lucky last year doesn't mean he wasn't valuable. He was very valuable. It's just not likely he'll be anywhere close to as lucky ever again. Even if we project a 10% improvement, but a league average BABIP*, Santana's slash line would be .245/.290/.415. The average shortstop hit just .255/.310/.368 this past season, which makes Santana's projected .705 OPS slightly above average. Add in Santana's above average base running and what projects to at least average defense, and he looks like a valuable player even without unsustainable luck.
*A 10% improvement in one year isn't all that likely, but Santana is also likely to post at least a slightly above average BABIP over his career because he strikes out a lot (that's not factored into the stat) and he's fast, so he should get more infield singles than an average player, so it should even out or get close enough that the guesses aren't too far off.
Unfortunately, Santana's power numbers are also unlikely to remain anywhere close to what they were as a rookie. That would significantly reduce Santana's slugging percentage. In 2,400 minor league plate appearances, Santana hit .273/.317/.391. He was always younger than the average age for each league, so his numbers are a little more impressive in that context, but nothing in his minor league numbers suggest consistent power, even from gap to gap. He also didn't look very big, so it's highly unlikely a strength or conditioning program was responsible for the sudden power surge.
Had we taken say just 18 singles from the 39 hits Santana would lose with a league average BABIP, and then 13 doubles, 5 triples and 3 home runs, suddenly the .222/.262/.375 line drops to .222/.262/.296.Then giving the same generous 10% increase and again a league average BABIP, Santana's projected season would be .245/.290/.325, meaning he'd need to be an elite defensive shortstop to bring value, because his .615 OPS would be below average offensively. Santana's defense at short is really an unknown as he barely played their this past season, but most projections have him anywhere from below average to above average. As a good athlete with quick feet and a strong throwing arm, I'd expect him to be at least average and probably above average.
I'd prefer to see the Twins trade Santana at his peak value, ideally to a team who is still impressed by the shiny slash line and not what he's likely to do next season. Speculating on trades is silly, since it's impossible to know what gets offered, among other things, but if a team is willing to trade a solid, young, near-ready major league starter the Twins would be silly not to accept.
Santana was fun to watch this past season, but if the Twins want to get back to playing meaningful games late in the season, they need to start selling high. This off-season, trading Danny Santana would be doing just that.