I'm looking at it more from a perspective of what I think the Cowboys would do more than what I would do. Personally I wouldn't hesitate to go DT round 1, but it sounds like that is the furthest thing from the Cowboys thinking. I'll actually go as far as to say that its a total waste to go get a backer like Kenneth Murray if you wont put quality run defenders in front of him.
Tet isn't my top WR either, but hes the one guy who you can plug and play in the X role right away. Other WRs may be able to come in and have more of a direct impact right away statistically, but the downside is most of these guys are probably eating a lot of those slot snaps that you would like to get to Lamb. Tet can line up in the X on one side of the field, and you can get creative with bunch formations on the other and scheme Lamb the ball more IMO....I think his value at least early on will have more to do with schematics than overall skill/potential which will be appealing to a new underdog HC that probably feels some pressure to show his worth early on.
I'm just trying to guess along with what these guys are thinking.