Adam Thielen was WR 12 in standard scoring last season and WR 8 in PPR scoring. He performed as a WR one last year.
He also put up very good numbers in 2016 although this was more sporadic than what he did last season. He is good enough that when Diggs is out of the lineup Thielen can pick up the slack and do more when needed.
Adam Thielen is a very efficient WR as measured by yards per target and comparing that to other WR. Going into 2017 Adam Thielen had 10.1 yards per target. The only WR with higher yards per target was DeSean Jackson with 11.1
After last season Adam Thielen now has 9.5 yards per target over the last two seasons, which is still in the top 5 for all WR. He slips behind Julio Jones but ties AJ Green and Jordy Nelson and slightly better than Michael Thomas.
That is a good sign I think for Adam Thielen to be a top 12 WR again in 2018 as long as he gets enough opportunity. I think Thielens total targets may go down a bit this season (depending somewhat on if Diggs misses games or not) but he should still have 100-120 targets over the season or 6.25 to 7.5 targets per game and with that amount of opportunity you are looking at 950 to 1140 yards with 4-6 TD.
You have the possibility of an upgrade now that Cousins is the QB as I do think he is a bit better than Sam Bradford or Case Keenum (who played great for the Vikings last season). The volume that Thielen had last year 142 targets likely not there however.
I good argument can be made for Adam Thielen being more valuable that Stefon Diggs, however Adam Thielen is 28 years old for the 2018 season, he is in the prime of his career while Diggs will be 25 years old this season and still ascending. Seems pretty clear from ADP that fantasy owners value Diggs more than Thielen, but then I think Diggs has been a bit over valued for two seasons now while Thielen has been a complete bargain.
I think I would give pick 1.04 in a rookie draft for Thielen, maybe pick 1.03 as well. Hell I could see trading Guice 1.02 for him. Adam Thielen has been very good. I would still likely try to buy for less than this, but this is pretty much what he is worth right now based on his performance the last two seasons.
eta - Another way to look at it is
ZWK has Thielen as WR 16 based on projected career VBD and current age.