The coverage has said you need three guys to be out in order to not subject a replacement to DFA waivers later. Probably we don't want a third guy out more than we want to see Appel.
One thing I always forget about the roster though is that you can't actually put someone on the 60-man DL unless you max the 40. So if Moniak is still three weeks away - which he'd almost have to be at this point - he could go on the 60 if needed. It's a move that happens after you add who you need to add, not before.
Of course then he's also have to come off it.