I'm not an authority, but I think the way it works is you make the trade, there's some period of time that goes by according to your settings --- a day or two, probably, then if you actually get the guy on your roster fri/sat you can play him on sunday.
basically, you just need to get the guy in your active line up, like, 5 min before he plays in real life --- although, I think mon night games have to be lined up by sun midnight pst.
so, let's say he's rb and you're in the weird situation of having all thursday rb's, but you want to play the incoming guy ---- you'd just bench a rb and leave a spot open, then insert the incoming guy when you get him.