September MLB roster additions: James Paxton, Jonathan Schoop, others called up Tuesday

With minor league seasons coming to a close and September roster expansion in full-swing across the MLB, teams have been calling up players in droves for the final month of the regular season. It had already been announced that top prospects like the Reds' Billy Hamilton and the Tigers' Nick Castellanos would be called up, but other big prospects have also been brought up to the majors.

On the pitching end, four squads have called up top youngsters. The Mariners made perhaps the biggest splash, with James Paxton joining the rotation and scheduled to start Saturday. Paxton is one of the Mariners' big three pitching prospects alongside Taijuan Walker -- who was called up in late August -- and Danny Hultzen. Rated the No. 87 prospect in the MLB by Baseball America and No. 57 by MLB.com prior to 2013, Paxton has had a down year with a 4.45 ERA for Triple-A Tacoma.

The Cardinals strengthened their pitching corps even further with the addition of Carlos Martinez, rated the No. 48 prospect by Baseball America prior to 2013. Over 16 starts between Double- and Triple-A this season, Martinez has a 2.49 ERA. Though his peripheral stats could improve, Martinez is just 21 and is very projectable. He served a stint in St. Louis' bullpen earlier this season and had some struggles.

The Brewers called up one of their top pitching prospects in Johnny Hellweg. Hellweg, acquired from the Angels last year as part of the Zack Greinke trade, was recently named the Pacific Coast League's Pitcher of the Year after he posted a 3.15 ERA over 23 starts. Hellweg has had major control issues over his career, however. Those plagued him in a brief major league call-up earlier in 2013.

Erik Johnson is likely the best pitching prospect the White Sox have in their system. Though he has failed to crack any top-100 prospect lists over his career, all Johnson has done is put up excellent numbers. In just his third professional season, Johnson has a 1.96 ERA and 0.99 WHIP over 24 starts split between Double- and Triple-A.

On the hitting end, the Orioles brought up second baseman Jonathan Schoop. ESPN.com's Keith Law had Schoop rated as the No. 50 prospect, even after some struggles in 2012 that saw him post a .710 OPS in Double-A. Schoop has bounced back some this year with a .790 OPS and is the heir apparent to Brian Roberts at second.

The Athletics also brought up a hitter who struggled in 2012. Though outfielder Michael Choice hit 30 home runs in High-A in 2011 to put himself on the map, he dropped back to just a .779 OPS last year. This year, Choice's power numbers are similar to 2012, but his OBP has jumped up to .390 in Triple-A.