You can trade one of you Def teams for one of the FA teams, or a player that you won't be using (like your backup TE), or trade with another manager for a Def team to use.
BTW, for this reason it is useful to look ahead to make sure that everyone's teams have the correct selection of players. At one point I discovered that my two "favorite" RBs both had byes in week 10, so I was planning in advance for decent replacements for that week. And ironically I may be using both of the replacements this week!
Another tip that I discovered is that you can preset each week's lineup in advance. That way if something happens and you don't get a chance to manage your team, you won't be stuck with a team full of "byes" or backups. (When you make trades with other teams, or available players, you'll need to check again on how things sorted out.)
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Red Sox--2007 World Champions!
Celtics--2008 NBA Champions!
Isola is a magic island! All sorts of strange things can happen, and often do happen. -- arnie