MLBTR notes the PCL's transactions page, which says that the A's released Simon Castro. He'd been pitching at Nashville and the main thing in his stat line is that a lot of stuff was going on: 12 strikeouts in eight innings, but also 11 hits, six walks, and two wild pitches. He managed to log significant time for last year's bullpen, pitching the seventh-most innings (37) of any non-starter, as the team cycled through various mediocre options (Chris Smith, Zach Neal, Michael Brady, etc. etc.), traded their two best relievers (Sean Doolittle and Ryan Madson), and dealt with injuries (John Axford (six weeks), Doolittle (five weeks), Ryan Dull (nine weeks), Bobby Wahl (four months)). Castro was easily into the double digits of the relief pitcher depth chart, which looks something like this:

1. Blake Treinen
2. Yusmeiro Petit
3. Ryan Dull
4. Ryan Buchter
5. Danny Coulombe
6. Liam Hendriks
7. Santiago Casilla
8. Chris Hatcher
9. Lou Trivino
10. Wilmer Font
11. Emilio Pagan
12. Chris Bassitt
13. Josh Lucas
14. Bobby Wahl
15. Raul Alcantara
16. Simon Castro

The order of the guys ahead of Castro isn't really the point (e.g. Trivino might be considered further up the list right now) so much as the sheer number of players ahead of him for playing time. Even with the eight-man bullpen, and even with two relievers currently on the DL, and even with the struggles of Pagan and Hatcher, Castro just wasn't all that close to sniffing the big leagues. He still throws in the mid-90s, though, so he can probably latch on somewhere and keep trying to make things work at Triple-A for a while.