Frias was recalled from Triple-A Reno on Thursday.
Frias was sent to the minors at the end of March, and he posted a 3.29 ERA and 1.14 WHIP in 13.2 innings over three minor-league starts to begin the season. He'll provide bullpen depth after J.B. Wendelken (undisclosed) was placed on the injured list and Tyler Gilbert was sent down.