Frias takes the active roster spot vacated by David Huff, who was designated for assignment earlier on Wednesday. Huff went just four innings in a spot start on Tuesday night against the Mariners, a game the Dodgers came from behind to win 6-5. He was followed on the mound by Juan Nicasio, who threw 46 pitches in two innings.
"Part of that plan was knowing we were using Juan last night. Obviously we need enough to cover the game tonight and have enough protection for the rest of our guys," manager Don Mattingly said before Wednesday's game. "Carlos has been starting, he's built up to start. He has a lot of pitches in him, this would have been his day. He's going to be ready to go."
Frias was originally scheduled to start for Triple-A Oklahoma City on Wednesday night against Nashville, but Freddy Garcia started in his stead.
Mattingly was vague as to what role Frias would have beyond Wednesday night, or if more roster moves were coming.
"There could have been actually more [transactions], depending on how things went last night," Mattingly said. "Each game presents its own set of problems."
Frias, 25, made one start for Triple-A Oklahoma City, allowing three runs on eight hits in 5⅓ innings on Friday night, with five strikeouts and no walks.
The Dodgers are currently carrying 12 pitchers, including eight relievers, and 13 position players.