He is best suited for Nickel Corner, but he can also play Outside as well if need be & in this situation it's need be.
Ivey is the Boundary corner he plays the closest to the sideline on the opposite side of the hashes, meaning he covers the Flanker. Blades is the Field corner & has to cover the XWR/WR1, meaning he has to cover the most space from the LOS. The Field CB is always your best cover corner & has the toughest job because they typically have less help & are usually facing the best WR on offense.
And yes, Ideally the perfect replacement for Ivey would be Christian Williams, but Baker doesn't have much confidence in him yet & it appears he's still behind in terms of understanding assignments before they'll put him on the field full time.
The reason why I want Couch to replace Ivey is because if they're not going to play Williams then there's literally no one else besides him, because neither of the TF's are ready either.