I do, unfortunately, think Hightower may be done. D. McCourty, too. It's perfectly possible both of them round into form but they haven't given us any reason to be optimistic. If they are in fact done for, that's a mighty big blow to this defense to have two of their better players and leaders fall off a cliff in the same season.
I think a trade for a competent LB and/or S is borderline essential at this point. I'm more optimistic about the DL because I think Shelton and Brown are talented but under-performing, not to mention the possibility of promoting Valentine to the active roster; Flowers should be returning soon, too.
But, yeah, LB and S is worrisome. I want to see (a) trade(s) there. Tyrann Mathieu would've been a good signing this past offseason and he didn't command much money, iirc.