The "worst case scenerio" is pretty ugly.
If Bru, Rodney and Seau all were to retire, Samuel and Colvin were to leave via free agency, then without considering the draft or any other "adds" it might look something like this
DL - Seymour, Wilfork, Warren
OLBs - Vrabel, Thomas
ILB - Lua, Woods
CBs - Hobbs, Gay
S - Wilson, Merriweather.
That would be scary.
What I hope happens is Samuel stays (Pay the man please) and we draft a CB with pick #7 who is at least 6' tall. Hobbs becomes the Nickel, and 2 of the 3 potential retirees decide to lace 'em up another year.