By Marc Normandin

It's September, which means expanded rosters are a thing now. This in turn means that there are a whole lot of shiny new playthings for baseball fans to enjoy that have been called up in the past week plus, like Xander Bogaerts of the Red Sox, Billy Hamilton of the Reds, Taijuan Walker of the Mariners, and the Tigers' Nick Castellanos. Those guys are the future, though, and their time will come: let's instead focus on the present, where things are pretty amazing, because it's populated by the historically relevant sophomores, Mike Trout and Bryce Harper.

Maybe it's unfair to continually compare and contrast this pair, but that's what happens when pair of rookies as young and as talented as these two in 2012 come up and produce at the same time. This is, barring injury to either, going to be a battle of which of these two is somehow the more generational talent. Rather than pit them against each other here, though -- there will be plenty of time in the future to reflect on just which one of these is actually the better player, especially as Harper grows into his game -- let's instead use this time to simply remind everyone of just how ridiculous they are, and why they should remain in our sights even when the next wave of excellence has stepped through the door.

Trout is once again on a disappointing Angels' team, but it's certainly not his doing. He's making just over $500,000 as a second-year player, so he isn't eating up a significant chunk of the budget like a few recent free agent signings, and Baseball Prospectus, FanGraphs, and Baseball Reference all list him atop their AL wins above replacement leaderboards. Trout's bat has somehow taken another leap forward after a season in which he led the American League in OPS+, thanks to a batting line of .337/.437/.569. The difference from last year? Trout is walking more often, with a 14 percent rate that trumps 2012's 10 percent, and has also managed to cut down on his strikeouts: The league respects his bat, because they can't stop it, and it's given Trout more opportunities to reach base. A reminder: Trout is 22 years old, and plays his home games in a park that depresses offense.

Defensively, it seems as if Trout has taken a step back, at least if you believe the advanced metrics. In 2012, he was rated between roughly a win and two wins with the glove, but this time around, the loftiest total is about half-a-win, while the ratings go as low as a full negative win. There are plenty of reasons to explain the change, of course: defensive metrics are fickle, for one, and there is such a thing as a defensive slump. Maybe the muscle Trout put on between his first and second seasons slowed him down in the outfield just enough to make him good rather than fantastic, or maybe nothing has changed at all except our perception because of aforementioned fickle numbers.

Here's the thing, though: it almost doesn't matter what kind of fielder Trout is. Baseball-Reference penalizes him almost a full win because of his glove, and still has him worth over eight wins in an unfinished season. FanGraphs gives him that half-win in the field, and has him near 10 wins. Baseball Prospectus has him just a dash below average defensively in 2013, but credits him with 10 wins on the nose. Mike Trout doesn't even need to field well to be arguably the best active player in baseball, which tells you about all you need to know about his bat and his baserunning.

Harper's season might not seem as mind-blowingly impressive on the surface, but dig a little, and you'll be blown away. Like Trout, he's increased his walk rate significantly, and now draws a free pass over 13 percent of the time. His Isolated Power has jumped up considerably, going from .206 to .231 -- you might recall that Harper's rookie season ISO was already the second-highest of all-time for a 19-year-old, behind only Tony Conigliaro. As Conigliaro is likely the owner of the greatest-ever age-19 season, second is nothing to be ashamed of. While his age-20 campaign isn't quite as historically breathtaking, it's still top-10 according to Baseball-Reference, coming in ahead of Jimmie Foxx, and right behind both Conigliaro and Trout, as the ninth-best ever. In front of those four are a few guys you might have read about in a book or something, like Mel Ott, Ted Williams, Alex Rodriguez, Bob Horner, Frank Robinson, as well as a peer of Harper and Trout, Giancarlo Stanton.

He's pulled this off while playing with an injury since the end of April. Harper slammed into a wall on April 30, and it bruised his rib cage. Another collision gave him bursitis in his knee, which resulted in five weeks on the disabled list. He's dealt with a sore foot, a hip that's caused him to miss some time recently -- as Harper says himself, basically an entire side of his body hurts. Despite this, he's mashed when he's played, surpassing Ken Griffey Jr's age-20 OPS+ to this point, with a chance to crack the top-10 with a strong last few weeks: if this is what Harper does when he isn't feeling right, just imagine what he can do healthy and with another year of experience under his belt.

It's easy to lose sight of just how great this pair is at baseball, if only because the news cycle is always moving forward, and we're used to -- or, okay, real talk: spoiled by -- what these two can do to a baseball and a box score day in and day out. Trout can do everything, and do it exceptionally. Harper is a year younger and with less experience in the pros, but he's already put up historically significant, age-related numbers, and in spite of playing through and with injuries. It's easy to get distracted by this when Bogaerts hits his first home run, or Hamilton keeps pinch-running and swiping bags in meaningful September games, but there's plenty of room to appreciate the next wave while following along with the players that those newbies hope to someday become. Trout and Harper are special, and they're linked, and there's no end to the praise you can heap on the progress they've made in such a short time.

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Marc Normandin writes and edits for Over the Monster, a Boston Red Sox blog, as well as SB Nation's baseball hub. He's one of many behind the e-book "The Hall of Nearly Great," and has written for Baseball Prospectus, ESPN, and others. You can follow him on Twitter at @Marc_Normandin.