You're trying to have it both ways. You want to maintain the slot targets, but project the outside TDs. I don't think Diggs is a special red zone threat, but he'll clearly have more big play opportunities on the outside. Which is what the move to the outside was all about - big plays. It was essentially a promotion and it's worked. He's averaging 15 YPR and making explosive plays down the field, including TDs.
Diggs won a contested catch TD situation earlier this year. He isn't that big, so not a player you would expect to win contested catches in the red zone, but he
can do that. Career splits for Diggs are 22 targets 18 receptions 2 TDs. seven 1st downs. Scoring TD in the red zone has been a point of emphasis for the Vikings, but they have still ended up kicking a lot of field goals.
Diggs only has 11 TD so far in his 3 year career. So its not like he has been a prolific TD receiver. I think injuries, poor offensive line play and mediocre QB play are contributing factors in that.
Adam Theilen only has 7 TD over his 4 year career. One of those as a rookie. He is a bit taller than Diggs but its not like he is better than Diggs on a per opportunity basis. Theilen just scored his first TD of the season against the Browns. Wide open, one of the easiest TDs you will see a WR make, mostly because Keenum was rolling out and the defender came up to stop him from running. Keenum was a bit late with the throw too, as I think he could have hit Theilen earlier in the route, he was open for a pretty long time and just waited in the corner of the end zone for the ball.
Kyle Rudolph has been the main red zone target for the Vikings. One of the few things Rudolph is actually very good at. Using his height and rebounding skills to score TDs there. This is a role that Treadwell was drafted for but hasn't materialized yet. I think the addition of Michael Floyd was part of the Vikings trying to get better in the red zone as well.
Adam Theilen is very good Currently WR 5 in PPR leagues and less than 2 points behind AJ Green. Mostly with Case Keenum. Thats amazing. He is very solid and efficient. 9.5 yards per target for his career thus far. Diggs is at 8.6 yards per target, with more targets though.
Still think Diggs is the WR 1 for the Vikings, but both players are very close to the same value.
As far as their roles in the offense Theilen has played more from the slot this year. Both play all 3 WR roles depending on the play call. When Diggs has been out with injury Theilen slides into his role as the primary receiver. They run a lot of the same routes, I think Diggs runs his routes a bit better than Theilen does, he is more quick twitch, not to say that Theilens routes are bad. They are very good, just not as explosive as Diggs is.
The quick cuts Diggs makes are part of why I think he has had injuries. He puts a lot of strain on his body with some of the extreme moves he makes. Theilen wins more at the top of his routes, while DIggs wins earlier in the route on most plays.
Diggs is better with Bradford at QB than Keenum, because Bradford gets the ball out more quickly than Keenum, has better arm strength and accuracy. Bradford can hit those deep passes much better than Keenum, who really needs to get his whole body into those deeper throws.