Is this a Keeper league? IF so, maybe I can see it. If not, dropping Lee for an unproven player wasn't cool. As long as he is healthy, he's the better play. As for dropping Kaz for L. Castillo, that makes sense. It's about time for Kaz to get hurt.
I'd drop Matsui for Castillo - Castillo isn't flashy, but he's hitting in the two hole in a powerful offense, and he steals bags. Should be a good source of runs too. Do that.
Not a huge fan of dropping DLee for an unproven rook in a non-keeper.
I am a fellow Lee owner, but usually that Cubs lineup heats up when the weather improves, and it is still not great here. Did anyone scoop Lee up already? I would say don't be too proud to pick him back up.
I would make the swap of Matusi for Castillo and try to get Lee back. I know he is struggling, but he is too good of a player to just drop. Try trading him in some type of package so you at least get something back for him.