The game at the Hawthorns was goalless when Debuchy made a two-footed challenge on Claudio Yacob, actions that resulted in referee Lee Mason producing red.
Pardew had no qualms with the official's decision, stating he felt it was right to send Debuchy from the field of play for launching himself into the tackle.
The contest was decided late on when Saido Berahino converted from the penalty spot after keeper Tim Krul had brought down another of the Baggies substitutes Matej Vydra.
The Magpies tactician felt Newcastle could have gone on to claim all three points from their trip to the Midlands had Debuchy not been sent off just after the hour mark.
"West Brom let us have possession today. We had a lot of it and controlled the game to a degree. A few breakaways they had but other than that I felt confident we would win the game," Pardew told Sky Sports.
"But the sending off did change it of course. Unfortunately I think it was a red. Then it was uphill.
"After this amount of games we got tired. We had injuries as well. I had to keep shuffling our team. In the end something gave.
"I can't fault my players. They've given everything today. I'm just disappointed for the team today. Once instance has changed the game.
"Mathieu has made a decision there where he's taken off. You can't take off the floor with two feet in this division. He's won the ball, he's a fair player, but as I say you can't do that.
"I thought both teams looked a little jaded today. There wasn't that spark to the game. It looked like being a really tense affair. It did open up a little bit more in the second half, but the sending off changed that."