The 18-year-old hooker touched down late on at Headingley Carnegie Stadium as the Rhinos got back to winning ways with a 20-6 triumph.
"He was very good, he really took his chance," McDermott told Sky Sports News. "Robbie came in and had a huge impact on that game.
"The performance was good after last week. We've still got some tired men and have been through a very tough and busy period, a very intense period."
Leeds continue their Super League campaign next Friday with a trip to Warrington Wolves.