As Ed Bouchette points out on Twitter, the Steelers will place David DeCastro on IR on Monday. At that point they will bring somebody back that was cut or sign a player outside of the team.
There are several ways they could go with it but Trai Essex would be a logical option.
Maybe the sign a LB? That would greatly depend on how ready they think Harrison, Worilds, and Sylvester will be to start the season.
WR - Gilreath or Williams could come back and they could carry 5 after all.