The striker's effort was expertly saved by Costel Pantilimon when he was left one-on one with the Manchester City goalkeeper in Wednesday night's fourth-round tie at St James' Park.
It happened just minutes before City opened the scoring and Pardew said it was the turning point.
"If that goes in, it could've been different," he said. "I think it was obvious the first goal would be crucial and they got it."
The manager thought his side reacted well to their derby defeat at Sunderland.
He continued: "David Silva had a big influence when he came on but before that I thought we were the better side. I'm disappointed for the fans and our players because they put on a terrific show after the disappointment of Sunday.
"It was just a shame we couldn't get the goal we deserved."
Pardew added: "If you looked at us out there, I don't think you'd say we were in bad form.
"We played well enough to warrant a goal and I couldn't ask more from the players.
"The performance level was really high but it was a hard match so it's a shame we didn't get our noses in front."