You can change your captain each day as long as he isn't sent off or has already played, so mouldyuk is spot on, and pretty much everyone who has posted a team since I pointed this out is still making this mistake.
For instance I have Webo in my XI and Ronaldo benched to start, see how players get on and then decide the morning before whether to replace a player with Ronaldo.
Yep I've got Malouda in my starting XI with Kaka on the bench, so unless Malouda does really well, I will sub in Kaka nearer the first Brazil game. It's basically going to be about micro managing players pretty much most days, and gambling whether you think a bench player will get more than player who has already played.