The Padres made official three of the moves, optioning Castellanos and Jackson to Triple-A El Paso and reassigning Dickerson to Minor League camp.
For the two southpaws, Monday all but surely marked the end of their time with San Diego. Sipp can opt out of his contract if he's not on the Major League roster, and Schuster was a Rule 5 Draft pick acquired from Houston through Arizona, meaning he'll go through waivers before being offered back to the D-backs for $25,000.
In camp originally to compete for a left-handed-specialist role out the bullpen, neither Sipp nor Schuster experienced much success this spring vs. lefties, as batters from that side of the plate hit .455 off Sipp and .400 off Schuster.
That means Alex Torres will probably be the Padres' sole left-handed reliever. Robbie Erlin could be a second, but he's more likely to fill the fifth slot in the club's rotation. The competition is down to Erlin or the less experienced Matt Wisler for the job.
Black said Erlin threw 55-60 pitches in a Minor League game a few days ago in Surprise and will be stretched out to a pitch count of 75-80 when he throws next on Wednesday.
Monday's news also appeared to strengthen the case for right-hander Donn Roach to secure the club's final bullpen spot and share long-man duties with Tim Stauffer should Erlin make the rotation.
Gonzalez, Quackenbush and Boyer will still stay with the Padres for the time being and travel with the club to San Diego later this week for the two exhibition games vs. the Indians.
Black also mentioned the Padres are "still talking about" keeping three catchers on their roster. Nick Hundley owns the only certain spot, while Rene Rivera and Yasmani Grandal are the question marks.
There was a time when Grandal (knee) wasn't expected to be healthy enough for Opening Day but as the spring has progressed, so have his chances at being ready. He has appeared in five games thus far and is scheduled to catch all nine innings in a Minor League game Tuesday.
Tyler Emerick is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.