First, Fletch said he wants to leave and Marv seems to be building for youth so I don't see him throwing bags of money at a 32 year old MLB. Therefore I'm assuming he's gone.
Now, onto the rest of it. Say Marv decides not to re-sign Nate and we go after Briggs and Samuel. We get outbid in both cases. Meanwhile, Nate leaves. In the draft, both Okoye and Willis are gone before our pick.
Basically, this means we will go into next season trying to plug our existing defensive holes plus the loss of Nate and Fletcher with the guys on our roster, whatever we can scrounge together in the draft and maybe a few FA "role players" (like the ones that worked SO well for us last year- Price, Royal, Reyes, etc).
(BTW, I don't think the loss of Nate will hurt us too bad IF and ONLY IF we fix the pass rush and run D to take pressure off the corners).
Bottom-line: we are in desperate need of a playmaker or two on D, and there's a good chance we won't be able to get one this off-season. If that happens, we'll be lucky to go 7-9 next year.