That cannot be done. Doing so would leave Barcelona with 248 passing attempts. Rule 4.3 requires a team to have at least 400 pass attempts on their roster at the start of each game.
This rule makes sense at start of season, but I can't understand its meaning starting Week 8 as Keenum has already used all available pass attempts. However I'm changing my moves as follows:
Barcelona signs: 1. QB Brandon Allen 2. WR Duke Williams and drops: 1. WR Justin Watson 2. QB Tyrod Taylor
The rule, I believe, to keep teams from trading away players and not leaving themselves enough players at any particular position, pass attempts, receptions, etc., whether intentionally (tanking) or unintentionally (oversight).