It's not a matter of them saying "we need to go get a DT to replace Donald in round one." But his departure, along with Jonah Williams leaving, creates a lot of snaps and production that needs to be backfilled. And guys like Desjuan Johnson can only be counted on so much to take on bigger roles.
Personally, if they do take one of the top DTs (Murphy or Newton), I'd prefer it be after a trade back 3-5 spots. But regardless, Edge, CB, WR, OT and now DT are all on the table as far as first round picks. All would make (longer term) sense.