A gassed D that is on the field too long tends to fade toward the end. We saw that fairly often last season.
We also saw injuries in the secondary leave the D vulnerable to the pass, especially when the pass rush is weak to begin with, and gets weaker as the game ppogresses.
That said, I winced for the D when they'd manage a turnover or force a punt after just a few plays, and then had to go back on the field again - after the O only managed a three-and-out, or maybe five plays in what could have, should have, been a momentum changer.
It is to Morris' credit that he kept the D playing - through the notable but overlooked injuries on the D side of the ball, and the erstwhile demoralizing, woeful performance of the offense.