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Jeff Roberson/Associated Press

No. 20: Paul DeJong, SS, St. Louis Cardinals

Opening Day age: 24

Had it not been for Cody Bellinger, Paul DeJong would have cake-walked his way to the 2017 National League Rookie of the Year award. All he did was play a solid shortstop while blasting 25 homers with an .857 OPS in 108 games.

This performance seemed to come out of nowhere, but it was actually a realization of potential that had always been there. DeJong has the ideal swing for power, in that it keeps the ball off the ground and makes liberal use of his pull side. Factor in his steady glove, and you get a capable two-way shortstop.

No. 19: German Marquez, SP, Colorado Rockies

Opening Day age: 23

German Marquez didn't have as good a 2017 season as teammate Kyle Freeland, who only made it as far as the honorable mentions of this list. But between the two, Marquez is the bigger threat to get better.

He was plenty good as is in 2017, finishing with a 4.39 ERA (which equates to an above average 114 ERA+) over 162 innings. The fact that he got away with throwing pitches in the strike zone more often than any other pitcher is a testament to his mid-90s fastball and sharp curveball. Hitters will be in for an even rougher time if Marquez decides to make it less easy for them.

No. 18: Ian Happ, CF, Chicago Cubs

Opening Day age: 23

The awkward part of Ian Happ's inclusion in this list has to do with his playing time outlook. It's hard to call him a regular when he's projected to split time at various positions in 2018.

All the same, he made it quite clear in 2017 that he's an up-and-coming star. Despite a swing-and-miss issue that produced an ugly 31.2 K%, his power and patience led the way to an .842 OPS and 24 homers in 115 games. He's also a good enough athlete to handle multiple positions, including center field.

No. 17: Luke Weaver, SP, St. Louis Cardinals

Opening Day age: 24

Although Luke Weaver didn't find much success in his first taste of the majors in 2016, he did demonstrate an ability to overwhelm major league hitters by striking out 45 of them in only 36.1 innings.

He kept right on doing that in 2017, whiffing 72 batters in 60.1 innings. Even better, he was able to avoid walks and home runs this time. It helped that he boosted his average fastball to 93.2 mph, but more so that he added a pretty good curveball to go with his excellent changeup. With his experience and these weapons at his disposal, he's ready for the next step in 2018.

No. 16: Lance McCullers, SP, Houston Astros

Opening Day age: 24

Lance McCullers' primary goal for 2018 should be staying healthy. His secondary goal should be harnessing control that, while mostly fine, tends to come and go.

However, this isn't a guy who's gotten to a 3.60 major league ERA by accident. He boasts an electric mid-90s fastball and the hardest curveball thrown by any starter, as well as a changeup that's more than just a show-me pitch. These things make both contact in general and hard contact tough to come by.