It is derivative, in the sense used by the film/TV industry. You may not be aware but US TV is written work-for-hire, meaning that the writer gives up their rights. In this case, anything written or developed for the ER Pilot would be assigned to WBTV and be subject to their contractual arrangements with Crichton. Anything derived further from that material would also come under the same arrangement.

The Crichton position is that WBTV on the sly tried to shift work product from under the auspices of their arrangement and into a “new” production, to the loss of Crichton.