Doctors shared a significant update on Thursday, stating the 24-year-old had "won the game of life" in further explanation of a Bills statement that declared Hamlin to be "neurologically intact".
The quick actions of Kellington to administer CPR prevented a tragedy and McDermott took a moment to hail his actions in the team's first media availability since Hamlin's collapse.
"To give it context, Denny is one of our assistant trainers. Just like our team, their team go through mock exercises for these sorts of things, but we're not around for those. As they say, practice pays off," he told reporters.
"For an assistant to find himself in that position and take charge and step up like he did, it's nothing short of amazing.
"The courage that took, you talk about a real leader and a real hero, for saving Damar's life. I admire his strength."
The Bills are unlikely to complete the suspended Week 17 clash against the Bengals but are back in action this weekend against the New England Patriots as the regular season comes to a close.
Playing so soon after such a traumatic event will be a difficult scenario, but McDermott said Hamlin's father, Mario, had told them it is what his son would have wanted.
"I feel strongly that as his dad Mario had mentioned to us, that this is what Damar would have wanted. We owe that to Damar and to his family," he added.
The Bills sit 12-3 for the season, already clinching the AFC East, but have an outside chance of securing the No. 1 seed in Sunday's finale, with the NFL's decision on the suspended Bengals game pending too.