So no O-line, no running game, no TE (or slot receiver) and DT playing hampered and you expect a 1st year starter to carry the load (not even Brady was able to do that).

Add to that his injury and a scheme that is antiquated and took too long for plays to develop (btw, Siemian had the 8th highest 4th Quarter QB rating - which shouldn't be surprising as that's when Trevor was no longer "shackled" by the scripted plays).

How did Dak do last weekend when he no longer had a run game to rely on; and the Cowbys have a way better pass protecting O-line than what Trevor had last year.

Schofield - gone.
Sambrailo - gone.
Stephenson - basically a back-up to the back-up at the tackle positions.
Garcia - wouldn't be the starter if there was a better option.

The Broncos are 8-1 when Siemian attemps 35 or fewer passes (with the one loss being to KC, a game they should have won) and 7-1 when CJ plays (the one loss there being the Chargers game when Trevor came back too early from his injury).