If another receiver is added, then we will cross that bridge when we come to it. For the time being though, let's evaluate Moss' signing on the assumption that the receiver corps is now set.
The lack of a major free agent addition puts tremendous pressure on Leonard Hankerson. Hankerson doesn't need to just break into the top three - in order for me to feel good about the receiver crew he needs to grab the starting split end job and hold it. This is mostly because Santana Moss is still very good as a slot receiver, and it's vital that the rest of the receiver group is strong enough to keep him there.
Two receiver sets need to have Hankerson out wide at X providing chain-moving possession receiver production, while Santana is at Z to be put in motion and create matchups. In three receiver sets, Santana can slide in to the slot while Anthony Armstrong comes in at Z (as happened last year). If the two receiver set consists of Moss and Armstrong - i.e. Hankerson isn't yet ready for full time work - then someone is going to be working in a position that does not maximize his talents. And the three receiver sets will be harmed by having an inadequate player at X (also like last year, when Santana did well from the slot but the passing game was inhibited by having to put Joey Galloway or Roydell Williams out wide).
I have made the case myself that Armstrong may have a better chance than most to contribute in the short term. And while I'm glad to have him, I can only talk myself into cautious optimisim rather than certainty that he will be game ready in week one. He may be better prepared than most rookies, but he's still a rookie receiver who lacks the benefit of any offseason activities to learn the offense.
The only other option on the roster that could keep Moss and Armstrong in their proper roles is Malcolm Kelly. But as I have said before, until we see him on the field for multiple consecutive practices, not to mention games, Malcolm Kelly does not exist. At least not for roster planning purposes. If he manages to stay healthy and shows that he has developed his skills to a professional level despite all the downtime, then that's a nice little surprise that we can feel good about. But until that happens, any depth chart talk must ignore him.