Warren was drafted to be a NT, but they quickly learnt that he couldn't play that role and moved to DE.
Sorry to nitpick, but this simply isn't true. Many people assumed the Patriots wanted him to be a NT because that was a need, and Ron Borges said it repeatedly to make it look like the Patriots had failed with the pick, but, in fact, Belichick said from the day he was drafted that they thought of him as an end first, and it was reported by the objective (non-Borges) media that from the first day of rookie camp Warren was lining up at LDE and not NT. Yes, like all DLs, they tried him at different spots through training camp, but LDE (backing up Bobby Hamilton that rookie year) was always where he got the most snaps.