Sam Vesty admitted Saints were 'way off' the standards they expect - despite securing a 47-20 win against Leicester Tigers on Saturday afternoon.
The black, green and gold bagged a Premiership Rugby Cup semi-final spot thanks to a seven-try success against an inexperienced Tigers team at Franklin's Gardens.
Cobus Reinach registered twice, with James Grayson, James Fish, Dom Barrow, Heinrich Brüssow and Tom Collins also on the scoresheet.
But attack coach Vesty felt Saints were far from at their best, especially as Tigers had recovered from 26-10 down to cut the gap to six points with 11 minutes to go.
"It was a very imperfect display," Vesty said.
"We're not happy with our standards of what we were trying to do, but we did score 47 points against Leicester and we're through to the next round.
"What more could you ask for? You'd take it at the start of the day, but in terms of performance, we were way off.
"All we ever ask is that we do the processes of what we say we're going to do, and we went away from that standard we grade ourselves on.
"The disappointments aren't the dropped balls, they're the bits where we don't do what we said we were going to do.
"We have just scored lots of tries and given ourselves a chance of getting a home semi-final so I'm not trying to dampen that, but it is the right time to get the right standards out of us.
"We're a little bit disappointed but we'll take the points and the win."