The derby is a currently scheduled to be played three or four days before a potential champions league quarter final tie. It would be ridiculous to pick a full strength team in that game if the league is won. Jeopardising our advancement in a major trophy all for the sake of getting one over on them in a dead rubber of a fixture just plays into the notion they're more important than they are.
Not only that, the first string players would be backing out of every 50/50 challenge and conserving as much energy as possible in that game so picking a full strength side wouldn't really result in a full strength performance.
Ok pick the EDS