Seven April callups who made a big impact

As you might have heard simply by sticking your head out the window and hearing the wail of the wind, the Braves have called up MLB Pipeline's No. 2 prospect Ronald Acuna to make his Major League debut against the Reds on Wednesday night.
Acuna is considered one of the top hitting prospects of the past several years, and he instantly becomes one of the must-watch players in baseball. He's also another example of a relatively recent phenomenon: The April superstar callup.
Either because of changes to the Collective Bargaining Agreement or because teams want players to have an extra bit of seasoning in the Minor Leagues (or both), MLB teams only started bringing up their best players a couple of weeks into the season this decade. Obviously, teams call up players from Triple-A all the time; there were nearly a dozen Tuesday. But these top prospect callups have become commonplace.
Albert Pujols made his MLB debut on April 2, 2001 (Opening Day); Acuna is showing up with the season a month old, and his Braves already 22 games in. But Atlanta is only 3 1/2 games out of first place in the National League East and a game out of the second NL Wild Card spot; all told, not a bad place to be when you're adding the top hitting prospect in the game.
Let's take a look at past April callups this decade to see how they did when they were promoted to the Majors, and what happened soon thereafter. You'll note they span back to one particular superstar.
Cody Bellinger, Dodgers
Callup date: April 25, 2017

Bellinger was MLB Pipeline's No. 10 prospect when the Dodgers called him up after multiple injuries to their outfielders, even though he'd primarily played first base in the Minors. He batted eighth and played left field against the Giants' Ty Blach.
Bellinger popped out in his first at-bat, then in the seventh inning, he was awarded an honor that's rather rare for a player making his MLB debut: An intentional walk. His first Major League hit came in the ninth off Neil Ramírez. The lefty then went 0-for-his-next-7 before going nuts -- he hit two homers against the Phillies and never really stopped, bashing 39 long balls en route to running away with the NL Rookie of the Year Award.
Kris Bryant, Cubs
Callup date: April 17, 2015

Bryant's start to the season in Triple-A was well documented and hotly debated, though ultimately it was, of all things, an injury to Mike Olt (now playing for Boston's Triple-A affiliate, Pawtucket) that got him the call. Instantly put in the cleanup spot, Bryant went 0-for-4 in his first game, but he had two hits in his next.
It took a while for the power hitter to get going, though; Bryant didn't hit his first home run until May 9, his 21st big league game. He then hit homers in two of his next three contests and finished with 26 for the year, helping lead the Cubs to their first postseason series victory in 12 seasons. It would get even better the year after that, as he won the NL Most Valuable Player Award.
George Springer, Astros
Callup date: April 16, 2014

Springer wasn't quite as highly regarded as some of the other prospects on this list; he was ranked No. 20 in all of baseball by MLB Pipeline. Houston called him up because it wasn't getting much offense, but it took him a while to provide help; it would take Springer a month to get his average above the Mendoza Line and even longer than that for his first homer.
Springer figured it out, though, and has been a lineup mainstay ever since. He also ended up winning the 2017 World Series MVP, so he's got that going for him. Carlos Correa wouldn't be called up until June 8, 2015, not that it stopped him from winning the American League Rookie of the Year Award anyway.
Anthony Rendon, Nationals
Callup date: April 21, 2013

It's funny to think that all people could talk about when Rendon was called up was "he's stuck behind Ryan Zimmerman." Rendon ended up being sent back down to Triple-A Syracuse after just eight games, but he was called back up because Danny Espinosa was struggling at second base. Rendon hadn't played second base regularly since Little League, but he was installed there somewhat out of desperation. Because he's Rendon, he started hitting; and once Zimmerman's throwing issues necessitated his move to first, Rendon took over the job for good. He's since finished in the top six in the NL MVP Award voting on two occasions (2014, '17).
Nolan Arenado, Rockies
Callup date: April 28, 2013

(It's amazing how many of these first hits are of the infield variety, isn't it?)
Arenado is a star now, but when he made his MLB debut, he was somewhat of a fallen phenom: After spending time in the MLB Pipeline Top 20 years before, he had in fact been left off some Top 100 lists entirely.
Thus, Arenado was more a desperation callup for the Rockies, hoping to catch fire from a guy who happened to be hot at Triple-A Colorado Springs at the time. He was wobbly his first season -- he actually had an OPS-plus under 100 in 133 games his rookie year, a below-average hitter -- but ended up figuring it out.
Mike Trout, Los Angeles Angels
(Second) Callup date: April 28, 2012

All right, all right, so this one is cheating a little bit. After all, Trout famously made his MLB debut in 2011, when he hit .220 in 40 games and made everyone wonder whether he was all he was cracked up to be. He actually spent the first 20 games of '12 with Triple-A Salt Lake, where he hit .403 and looked like he could eat the whole league alive in two huge bites. So the Angels called him back up, the same day as Bryce Harper as it turned out, and he immediately became the best player in baseball, a designation he still holds today. He has still yet to win a postseason game, however.
Bryce Harper, Nationals
Callup date: April 28, 2012

Harper was famously only 19 when the Nationals brought him up from Triple-A, where he was hitting only .243 with one homer in 21 games for Syracuse. His first hit was classic Harper: A drive over the center fielder's head, a dead sprint toward second base, even knocking his helmet off along the way.
It would be a couple of weeks until Harper hit his first homer, but he held onto his spot in the lineup all season, and really ever since. Perhaps most tellingly: Before Harper arrived, the Nationals/Expos had made the postseason just once in their 43-year history. The franchise has reached the playoffs four times since -- even if it still hasn't won a series.
The Braves own the sixth-longest postseason drought in the Major Leagues; They dream of Ronald Acuna being their Bryce Harper, in more ways than one.