Nieve was 2-5 with a 5.72 ERA over 36 games (seven starts) for the Express. Cassel compiled a 9-5 record and a 3.69 ERA over 19 games (17 starts). Although Cassel has extensive experience as a starter, both he and Nieve will join the Astros' bullpen for the foreseeable future, as will Arias, who was 1-0 with a 1.52 ERA and one save over eight appearances (three starts) for the Express this year.
"Arias will be just like Nieve and Cassel -- middle innings, long relief or a situational need," Astros manager Cecil Cooper said. "If we need to have someone stretched out to two or three innings, he's capable of doing just that."
Arias was claimed on outright waivers from Colorado on July 31. He posted a 2.63 ERA over 12 big league appearances for the Rockies, and he also pitched in 30 games for Triple-A Colorado Springs in '08, finishing with a 3-4 record and a 4.73 ERA.
Maysonet, 26, is making his first appearance on a Major League roster. In 117 games for Round Rock in 2008, Maysonet has hit .271 with six home runs, 34 RBIs and 24 doubles. He was selected in the 19th round of the June 2003 Draft and played collegiately at Delta State (Miss.).
Cooper said Maysonet may start later in September, but for now, he'll be used as a utility player.
"He'll get a lot of play at short and second," Cooper said. "He's really a gifted defensive player. He can play all over the field."
Towles, 24, was hitting .304 with seven home runs and 28 RBIs in 48 games for Round Rock this season. He has played in 50 games for the Astros in 2008, hitting .143 with four home runs and 16 RBIs.
Cooper said Towles and Humberto Quintero will share the majority of the catching duties, and Brad Ausmus will continue to work with Roy Oswalt.
"I'll probably end up trying to split them where they each catch a couple of times out," Cooper said of Towles and Quintero.
Towles has appeared in 64 Major League games for the Astros during the last two seasons (2007-08) and has hit .194 with five home runs, 10 doubles and 28 RBIs.
The Express concluded their season Monday afternoon with a 1-0 loss to Albuquerque that lasted a quick one hour and 35 minutes.