If you're worried about someone leaving, get an old jersey. Honestly, though, I wouldn't shell out a lot of money for the jersey of a former player unless it has a signiature.
As for current players, if you are really concerned with tenure, Roy, Witten or Newman are probably your 3 best options. Yes, yes, yes, I know that 2 of the 3 of them are going to become free agents, but also note that they are our 3 cornerstone, homegrown talents. I would bet that at least 2, if not all three, will get to stay.
They have been productive, and if there is one thing we all know, Jones takes care of his own. These guys bring revenue to the team - Jones knows that.
Just get the Roy - he isn't going anywhere.