Of the players cut, the most significant departures were Herrera and Castillo.
Castillo, who had a 3.60 ERA in three games, was identified as someone who could leapfrog from a second-tier group into contention for the rotation as he impressed the club.
"Probably the first thing is that he hasn't done anything to take our optimism away," Reds manager Bryan Price said. "What we had to do, considering innings and opportunities we had for length, is we had to pare it down to who are the most likely.
"We had to rank them in order to give them regular time and regular innings. I just felt with his limited Double-A time that it made more sense for him to start the season in player development and maybe be one of those quick-jump guys should we need him early in the year. That was a tough decision."
Herrera was batting .308 with a .438 on-base percentage and five RBIs in 14 games, all as a designated hitter. He suffered an injured right shoulder early in camp, which has prevented him from playing second base in games. Herrera's throwing program has been progressing, however.
The Reds will stop using a DH beginning with Friday's game vs. the Indians.
"I'm going to have [Brandon] Finnegan hit, so the DH at-bats go away," Price said. "A periodic pinch-hit at-bat compared to getting maybe three, four or five at-bats a day in player development, and continuing his rehab and eventually getting back into games as a defender is something we can monitor and manage a lot more easily in the Minor Leagues."
Mark Sheldon has covered the Reds for MLB.com since 2006, and previously covered the Twins from 2001-05. Follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.