They chose to wait until today to do so.
The team confirmed DeCastro will no longer be on the active roster, freeing up a space for, presumably, CB DeMarcus Van Dyke. BTSC wrote yesterday the Steelers were going to sign the former Raiders third round draft pick, and the question was what move they would make to free the roster spot.
The team announced the signing of Van Dyke, and he will be fully eligible to play in Sunday's game. Whether he will is a different story, but it seems logical to think he could participate in special teams duties if needed.
Clearly, placing DeCastro, who dislocated his knee cap and tore his MCL, on the IR (with designation to return) was the cleanest option. The addition of Van Dyke will give the Steelers nine active defensive backs for Sunday's game - Van Dyke, Robert Golden, Troy Polamalu, Ryan Mundy, Will Allen, Ike Taylor, Keenan Lewis, Cortez Allen and Curtis Brown.