I agree that the defense should get to choose who to guard. The ideas I had thought of on how to combat that problem were not that good, but I'll just throw my thoughts in there. For example, I play vs. a team in my league who has a great guard who scores a lot (Jorge Kane) but he bounces around from SG to SF. My best OD is Nate Byrne. My thought was in pre game tactics. Have "Byrne guard ... Kane".
Problem here is Kane might not play, so I thought of this...
Maybe instead of lineups being set 15 minutes before tip off, a "line up card" must be completed an hour before tip off which only has the names of players that are suited up for the game and the position of the starters (so you know where each guy is playing). This could give a 45 minute buffer/defensive adjustment option (ie lineup card at 7, tactics/defensive adjustments 7:45, tip off 8) where managers could specify defensive assignments.