Slot WR is the safety blanket for the QB in this offense. Lots of single coverage against LBs and less athletic DBs like nickels and safeties playing off coverage. What you want there is a reliable, veteran player who you can count on to run the right route and catch the ball. Also important for that guy to be smart in order to sight adjust to blitzes and to find the soft spots in the zone. Duvernay is that kind of experienced, smart player. He's supposed to have the best hands of any Texas WR. He won't be as tough as LJH moving the pile downfield after the catch but he's a guy the staff probably trusts to convert 3rd and 5-7 yds.
At Z you need a guy fast enough to stretch the field and his hands have to be good enough to credibly threaten the defense deep. Medium fast is good enough if the line can protect against a 4 man rush. Herman and Co have recruited a lot of fast guys and some of them have good size as well. Eagles may not be quick but his top end speed is enough to do that job and he seems to have good hands as well. Lastly if Sam is still working on the accuracy of his deep ball, the bigger catch radium might help and the extra size to basically post up the defender on a back shoulder throw doesn't hurt either.