I have not heard anything new about this "signing". Has he taken the physical? If so, I hope they took lots of advanced images of his f***ed-up knee and that someone competent reviewed them.
If he does pass or has passed the physical, is he ready to start camp and participate in normal ST activities right away? He was still rehabbing in December so it could reasonably be assumed that he is not ready yet.
If that knee is not ready to go now I say pass on Nunez. We need a healthy bat and placeholder for Pedroia now.
Of course, if all goes well, he comes on board and rakes until Pedroia returns and rakes himself. Nunez can spell not only Devers at 3rd as Moon has suggested, but he can give X a blow at short. X has stated as recently as this week that he tires in the second half. Nunez can help keep him fresh if things go well. Of course Nunez can sub for Pedroia as needed too. I don't want to see him in the outfield unless it is a true emergency.
Overall, Lots of things have to break right for the Sox.
We'll see.