Jim Mallinder was left to rue some 'crucial decisions' that went against Saints as they suffered an 18-15 defeat at Sale Sharks on Saturday evening.
Mallinder's men had been 12-3 up during the first half, but the sin-binnings of Michael Paterson, Jamal Ford-Robinson and Ben Foden eventually cost Saints.
Foden's yellow card, for what was viewed as a deliberate knock-on by referee Karl Dickson, who was taking charge of his second Aviva Premiership game, looked particularly harsh.
And Mallinder felt there were further reasons to be disappointed.
"A few decisions didn't go our way - three yellow cards and they scored all their points during the 30 minutes we played with 14 men," Mallinder said.
"We will look back and the ref will look back and learn a lot because he's young, he's inexperienced and hopefully he'll learn a lot from watching it.
"If you look for the build-up to Paterson's (yellow card), Foden was clearly on the ball and it should have been a clear turnover for us.
"The (Faf De Klerk) try they scored, Jamie Gibson was being held back and the referee said it was Gibson who initiated it, but I don't know why our flanker would initiate holding on to a
scrum when we're in defence.
"There were some crucial decisions and in a very close game like that, it's the difference between winning and losing."
Saints had the chance to level the scores with three minutes to go, but they turned down a kickable penalty in a bid to go for the full four points.
And Mallinder backed the decision of his players, who twice saw attacking positions slip from their grasp in the closing moments of the match.
"We wanted to be brave really," the Saints boss said.
"You could get an extra point, but we wanted four points.
"We left it to the players to make the decision and we said we'd back them either way.
"It was good that they went for it and unlucky that we couldn't get the win."