I've seen a lot of people saying that EA won;t be able to make you use a certain legend in an SBC (if they are required) as they can't differentiate them and can't put a requirement as "Use 90 Henry", for example. That's completely false, they could easily make you use a certain legend. We've seen they absolutely cannot use a players name, hence the "Senegal player from Liverpool" this year and the "Swedish from Utd" last year. People have been saying stuff like "They can only put you need a French legend, I'll use Pires".
It's incredibly easy for them to manipulate it and that's why, I believe, they've introduced the blocks in SBC's this year. We'll use Henry as an example again, they could fill the SBC with 90 rated french guys and then leave the LW spot open and say the team rating needs to be X which only using the 90 Henry will be able to get you. Now, to every rule there's always an exception. Brazilian attackers like Ronaldinho and Pele both have 91 cards but the prices on these guys are already expensive and the cards are limited as is so you wouldn't be able to cheap out like you would with Pires for Henry