Skill-wise both Durant and Forbes are probably best served in man usage, of course neither have any ball awareness whatsoever (they both try to faceguard instead of getting their heads around) but physically they can both play that sticky style.
That said Durant has clearly improved a great deal in his zone work. Problem is the general lack of anticipation and awareness for both in zone. Neither really deduce and jump routes. Both are weak tacklers. So it's completion after completion, surrendering yards, with no punishment of QBs. And offenses like SF will wear them down.
It is possible both those guys end up losing snaps down the stretch as a result, once Spoon comes back. Because then you have three corners who are not liabilities in tackling (Spoon, McCreary, and Williams) who can sit back behind the rush, come up and make quick tackles, maybe break up the occasional pass, and help this defense get more stops instead of surrendering yards until the field shrinks.
And on top of that we have a large safety who's been holding it down in the slot. So the aforementioned three are competing for two roles, and even the dime role has more competition. And that competition should push production in a good direction.