Morrison has been preferred and impressed in the FA Cup defeat at Manchester United to keep his place at the weekend.
"I thought Morrison was excellent against United and it was just a personnel decision," McDermott said.
"You live and die by that.
"I spoke to Pearcey on Friday, I said to him he needs to do better (for both goals against Stoke).
"He had to do better picking up Robert Huth for the set play and then when (Cameron) Jerome turned him for the second goal. He accepted that."