McNamara had virtually a full squad to pick from, with only the suspended Ben Westwood and injured Sean O’Loughlin not in action on the day, although Zak Hardaker and Tom Burgess turned out for England Knights rather than the full side.
After falling behind early on, England recovered to lead 14-12 at the break, but were kept scoreless in the second half as Ray Nasso levelled the score with a penalty, before Mantellato’s decisive kick.
McNamara said: “It was really disappointing. We didn’t play anywhere near what we are capable of, that was clear to see.
“We gambled on the long trip away, only got back on Wednesday, but we were fine and fresh and ready to go, so that’s not an excuse.
“I don’t think we were fully focused on the game. I suppose we’re glad it’s happened this week rather than next week. It’s now how we respond to that.
“There’s a lot of work to be done, but we’re confident you’ll see a much improved performance next week.”