By buying out Girardi, the Rangers also had another spot open up to protect a defenseman in the expansion draft.
Adam Rotter: The Rangers could conceivably sign Brendan Smith to a new contract before Saturday, when the protected lists are submitted, and protect him in the slot that Girardi was previously in. It was kind of expected that if Smith was going to re-sign with the Rangers it would occur after the expansion draft so that they could avoid Vegas potentially taking him and his new contract, but with that slot to protect another defensemen open, it makes sense lock Smith up if they can.
Otherwise they could protect Nick Holden with that new slot or potentially a new defensemen they acquire via trade. The Rangers have some options now after buying out Girardi but it would seem like the easiest would be to lock Smith up.