The German's mistake allowed Wilfried Zaha to grab a second goal for Palace, who would eventually win 3-2 at the Emirates.
However, Leno does not believe Mustafi should take responsibility for the loss.
"There are so many mistakes in the game," he said. "Every goal is about mistakes so it is not a big problem when someone loses the ball and we concede a goal.
"It's not a big problem because we are one team, we stay together so everything is easy."
Asked if Mustafi had apologised to him in the dressing room, Leno added: "No, we are one team. We win together and we lose together. It's not that one player has to apologise for a mistake.
"It was maybe a wrong decision for the second goal but you can't change it. Mistakes happen. I think there was 30 minutes still to play so we had big time still to win the game."