The move takes the pair off the team's 40-man roster, which now stands at 38 heading into next week's Winter Meetings in Indianapolis.
Newby was 2-3 with a 3.99 ERA in 42 games (two starts) for Double-A Mobile this past season and was impressive early on in big league camp last spring.
Carlin, meanwhile, hit .321 with 17 doubles, seven homers and 35 RBIs for Reno last year. He had 21 plate appearances at the big league level and hit .167.
The pair each have the right to refuse their assignment and elect to become free agents.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.