You don't have to worry about that somewhat underwhelming 16-15 start to the season, Cubs fans. Anthony Rizzo is going to start hitting. So is Addison Russell, and, one would assume, so will Kyle Schwarber, Willson Contreras and Benjamin Zobrist.
The rotation will be fine, and in case it's not, the Cubs have plenty of prospects left to go get Yu Darvish or Marco Estrada or whichever quality starter is available this summer. Heading into Tuesday's doubleheader in Denver, the Cubs were still projected to have nearly an 80 percent chance to win the National League Central.
It's fine. It's all going to be fine. The trophy is still very shiny.
But -- and you knew there'd be a "but" here -- it's at least worth looking into what else has been ailing the Cubs over the first month of the season, and one of the issues is from a somewhat surprising source. Their outfield defense, which was so phenomenal last year, has taken a big step back. Is this just a rough few weeks, or is it a sign of things to come? Let's see if we can figure out why.
The first thing to do is to quantify the idea that the Cubs' outfield really isn't performing like it did last year. And while making assertions based on barely more than a month of defensive data is always troublesome, what we have working in our favor is that a variety of different metrics all agree here.
Let's start with Catch Probability, which measures, based on distance needed and opportunity time, how likely a particular batted ball was to be caught. Last year, the Cubs' outfield was tied with the Royals for the second-highest percentage of catchable balls turned into outs, just behind the Rays.

This year? Well, that's gone from tied for second best (86 percent of catchable plays made) to tied for second lowest, 78 percent, just ahead of the Brewers. If we compare the year-to-year changes, the Cubs' outfielders have indeed taken the biggest step back, a drop of eight percentage points. This list makes a lot of sense with the Twins and Mariners near the top, given how much praise we heaped upon Minnesota's outfield this spring and that Seattle made a ton of trades to improve its fielding.
Not sure about Catch Probability yet? That's fine. Last year, the Cubs had the fourth-highest outfield Defensive Runs Saved; this year, that's 22nd highest. (And the only current Cub who rates as a positive this year, Jason Heyward, just landed on the disabled list.)
We can make it even simpler. Let's look at batting average allowed on non-grounder batted balls that stayed in the park and went at least 200 feet as an easy proxy for outfield play. Last year, the Cubs allowed a .335 average against -- second-lowest in the game; this year, they've a .422 average that's again the second highest.
So no matter how you slice it, fewer balls are being turned into outs by the Cubs outfield. Why? The most obvious reason is that the 2016 Cubs had a historically good defense, and whenever something that great happens, the safe bet is against it happening again. That's not to say it can't, just that it usually doesn't. There's more to it than that, though.

The players are simply not the same
Though the Cubs' infield and rotation are very similar to what we saw in 2016, the outfield changed considerably. Last year, the five outfielders with the most playing time, in order, were Heyward, William Fowler, Kristopher Bryant, Jorge Soler and Matthew Szczur. This year, that's Heyward, Schwarber, Almora, Jon Jay and Zobrist. Your milage may vary on how you feel about those changes; while Schwarber isn't in the lineup for his defense, neither was Soler, and Almora has a reputation as a strong defender. Regardless of whether this is better or worse, it's undeniably different.
Heyward played great defense last year, and he was doing the same before he got hurt in this year. What's interesting, however, is that Almora hasn't quite lived up to expectations yet. That's not to say the skills aren't there -- we do remember how he robbedMatt Adams last month -- but a year after catching 96 percent of the catchable balls that came his way, he's down to a more average 86 percent. As you can see, after letting only a few balls (in green) over his head last year in center, he's missed more catchable balls this year:
Gif: Albert Almora 2016-17 range
They haven't all been easy, of course, like when he raced to try to track down this Jose Osuna triple. This is just the sort of play you're accustomed to seeing him make.

It's worth noting here that a year after pushing Fowler deeper to get him in position to prevent dangerous extra-base hits, the Cubs' center fielders are playing shallower again. Last year, they averaged 319 feet from home plate, the 10th deepest; this year that's 313 feet, tied for 21st. Again, that's not so much "better" or "worse" as it is "different," and Almora has proven himself to be so talented defensively that it's hard to not see him performing better as time goes on.
Schwarber's been about what you'd expect, which is that he's a fielder with limited range who is occasionally capable of doing something great. Jay, however, seems to have taken a step back, as Catch Probability and DRS both agree here. This Billy Hamilton single, where you can see Jay misread the play and not get to full speed on a ball that's caught 88 percent of the time, is the kind of ball that the Cubs caught in 2016.

The pitchers have been slightly less effective
Of course, defense isn't just about the fielders. The pitchers play a role, too, and there's some evidence that they're just making things harder on their outfield this year. Remember, while the Cubs did get to the second-highest percentage of outfield balls last year, it was more about making all the plays rather than making the spectacular plays. Although they had the fifth-fewest "Five Star Catches" in MLB last year, they also had the second-fewest opportunities. The pitchers simply weren't allowing the kind of hits that lead to amazing plays, like this one from Heyward:

This year, Cubs outfielders have had 19 opportunities to make Five-Star plays, exactly the Major League average. And a big part of that is that the pitching has changed. A year ago, Cubs pitching -- based on a combination of exit velocity, launch angle and strikeouts -- allowed an Expected Batting Average of .229, the third best in baseball. That's saying that Chicago pitching was very good at suppressing the kind of contact that turns into hits. This year, that's .241, which is exactly league average. Doing anything at an "average" level isn't bad, it's just that it's well below "elite," which we saw last year.
As always, this is an accounting of what has happened, not necessarily what will happen -- though a minimum of 10 days without Heyward could lead to some outfield adventures. Almora ought to get to more balls. More of Bryant, who was a surprisingly decent outfielder last year, could help. The rotation looking more like it did in 2016 would help.
This is still a talented group. It's just not going to be historically good again. When it comes to incredible baseball performances, history rarely repeats itself.