Jim Mallinder expressed his disappointment after Saints just missed out on sixth spot in the Aviva Premiership.
Mallinder's men beat Harlequins 22-20 thanks to tries from George North, Alex Waller and Api Ratuniyarawa at Franklin's Gardens.
But it wasn't enough as Saints finished seventh courtesy of having won fewer league games than Quins this season.
That means Saints could face two play-off rounds later this month in their bid to secure Champions Cup for next season.
But they will definitely be in the Challenge Cup if Gloucester beat Stade Francais in the final of that competition on Friday as the Cherry and Whites would take the Champions Cup play-off spot.
Saints could have secured automatic Champions Cup qualification had they won by more than seven points or earned a try bonus point against Quins.
But they were unable to take late chances that came their way.
And Mallinder said: "It was disappointing at the very end.
"We scored some good tries, did some good things, but we had a bad start to the game, going points down in the first few minutes. You've got a big fight to come back from then.
"I was proud of the lads' effort, plenty of effort, but we're disappointed we couldn't get that fourth try.
"It's easy to say we needed a bit of composure. At times we were pretty composed, but at others we weren't.
"We had a fair bit of ball, we had our chances five metres out on a few occasions and just couldn't get over."