The Cobblers grabbed an early lead in the first half of Friday's 1-1 draw at Elland Road, but lost Giles Coke to a 27th minute dismissal for a second yellow card.
Andy Robinson's penalty restored parity, but Leeds were unable to find a winner on a frustrating night for the home fans.
"People will look at it and say 'doom and gloom' and it was a bad result, but the truth is when they were down to ten men they got bodies behind the ball and tried to frustrate," said McAllister.
"At times I thought our passing was excellent and some of our play was sublime. We created numerous opportunities to get a good ball in the box.
"As much as I'm disappointed, I won't lose sleep any over it.
"We're still in the FA Cup and there were loads of positives to take out of the game."