The book on Troy Tulowitzki has been the same for most of his career: The best shortstop in baseball...when healthy. Over the last six seasons, Tulowitzki has averaged just under 110 games played and logged over 550 plate appearances just once. However, in that same span he leads all shortstops in batting average, home runs, RBI, OPS, and wOBA. While many fantasy owners were reluctant to invest in his services given his health issues, others reasoned that 100 or so games from Tulo and the remainder from a stopgap was still enough to maximize production from the notoriously thin shortstop position.

In 2015, Tulowitzki played in more games than he had in four of the past five seasons, despite spending the bulk of September on the disabled list. The problem, then, was one we're not used to seeing from him - he was ordinary. Granted, a .280/.337/.440 line is still better than what most shortstops produce; we're pretty far removed from the days of Rich Aurilia posting Mike Trout-type numbers. But in order to justify his price tag and offset the seemingly inevitable DL stint, Tulowitzki has to be the elite producer we've come to expect. There are some troubling signs that he may not be that guy going forward.

It wasn't supposed to be this way. Tulowitzki had offseason surgery to repair a hip issue that had been bothering him for the last few years, and the hope was that would allow him to avoid injury while maintaining his usual stellar production. So what happened?

One of Tulowitzki's strongest assets throughout his career has been his control of the strike zone. In addition to a double-digit walk rate, he had struck out in only 16 percent of his plate appearances coming into the season. Both numbers went in the wrong direction, as Tulo walked only seven percent of the time and struck out three times as often. Each mark was easily a career-worst. He also posted the worst isolated power of his career (.160) outside of his middling sophomore season in 2008.

Let's look at the power drain first. Nothing really jumps out in his batted ball data. No drastic change to his profile, as he still hit plenty of line drives and made typically hard contact. Some will undoubtedly suggest that leaving behind the thin air of Coors Field played a role, but that convenient narrative doesn't really hold up. Tulowitzki's home runs were down before the trade, and Rogers Centre was actually more friendly to right-handed hitters than Coors this season. The league-switch theory doesn't pass muster for the same reason; these were full-season issues not brought on by the trade.

There are two plausible explanations, and one is more charitable than the other. Tulowitzki's BABIP in the second half was rather low at .268, despite a 25.5% line drive rate. That's partially explained by a spike in pop-ups, but given his .321 career mark, it seems reasonable to suggest that poor luck played a role.

The second and much more troubling theory is deteriorating bat speed. Tulowitzki has always feasted on fastballs; per Fangraphs pitch value metrics, he ranked behind only Joey Votto and Miguel Cabrera in performance against the pitch from 2009 - 14. In 2015, however, he was essentially an average hitter when pitchers brought the heat. Scouts also commented that his lower half explosiveness looked diminished.

A fluke? Perhaps. But Tulowitzki is 31 years old. He's got a lot of wear and tear on a body that was never exactly durable to begin with. And now he'll have to play half of his games in one of the two stadiums that still feature artificial turf. It's fair to wonder if Tulowitzki's decline phase is on the horizon. With the emergence of young shortstops like Carlos Correa, Francisco Lindor, and Xander Bogaerts, Tulo's reign as the consensus top player at the position may be at an end.

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