Magpies captain Shelvey was sent off for standing on Dele Alli's ankle four minutes into the second half with the score at 0-0, handing Spurs the chance to step up a gear and clinch a 2-0 victory.
Like Shelvey, who was banned for five games last season for racially abusing Wolves midfielder Romain Saiss, Alli has had his disciplinary problems in the past, but he kept cool and was rewarded with the opening goal, before Ben Davies wrapped up the win.
Benitez admitted Shelvey's moment of madness had cost his side any chance of emerging from a tough opening fixture with anything to show for their efforts.
Benitez also insisted Harry Kane should have suffered a similar fate for a first-half challenge on debutant Florian Lejeune.
Asked if he felt let down by Shelvey, the Spaniard said: "It's not me, it's everyone. We have been talking about that for a while. He knows that he had made a mistake, he apologised, but still we lost the game.
"Also, I would like to say that the tackle from behind by Harry Kane I think was worse, but the rules are the rules and we paid for that."