That was a horrible read by Prescott. Didn't matter what Dez did in that route there was safety help up top and it appears the QB thought Dez was in one on one coverage.
Oh well...move on to the next play and possession.
Look at tonight, and the last 2 weeks. His play has definitely taken a step back. All rookie QBs do this, even the great ones. It just took Dak longer than most to actually get there.