The 2018 Wembley winners raced into a 12-0 lead after 20 minutes of their quarter-final thanks to tries from Fouad Yaha and James Maloney.
They were still leading by eight points after an hour but the Red Devils struck back to level and won the game in extra time thanks to a golden-point try from Dan Sarginson.
McNamara said: “We’ve nobody to blame but ourselves. We were eight points up with 20 minutes to go and stopped playing.
“We stopped being physical, gave away a soft penalty and then a try at the end.
“We started the game reasonably well and were in control for 30 minutes but then let couple of tries in after missing one-on-one tackles.
“We gave Salford some momentum and they took that into the second half as well.”
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