The Cubs made the moves before Tuesday night's game against the Mets.
Catcher Eli Whiteside was recalled from Triple-A Iowa.
The 33-year-old Veras signed as a free agent over the winter and was projected to be the team's closer. But he struggled from the start and was 0-1 with an 8.10 ERA in 12 games.
With Hector Rondon back from paternity leave, the Cubs opted to part with Veras instead of sending one of their young relievers to the minors.
Cubs general manager Jed Hoyer says he was surprised that Veras didn't do better.
''There's nothing more volatile than a reliever,'' Hoyer said.
Castillo is hitting .242 with five home runs and 21 RBIs. He said he hurt himself taking a swing Sunday, and an MRI revealed inflammation in the ribcage.