The Chicago Cubs needed a roster spot to activate catcher Welington Castillo from the disabled list (rib cage inflammation), and it was pretty obviously going to come at the expense of fill-in back-up catcher Eli Whiteside. And, since Whiteside can’t be optioned back to the minors. The Cubs get a 40-man roster spot in the process, too, as Whiteside has been designated for assignment (Bruce Miles).
Castillo isn’t starting today, which he wouldn’t be anyway, since John Baker tends to catch Jason Hammel’s starts. But Castillo is back and available after missing the last few weeks.
Assuming Whiteside is waived, he could go unclaimed, and then could choose to accept an assignment back to AAA Iowa.