Managing your fantasy football roster in the playoffs is completely different than how you work things during the regular season. We've only got two weeks left, so you can think -- you should think -- with a short-term mindset. If a player has a pair of tough matchups in Weeks 15 and 16, then replacing him shouldn't be an issue.
If a player's no longer going to be usable for you, and if you don't care if that player is on someone else's team, then he should be dropped. You don't get a prize for having the best bench after Week 16's over.