Matt Ferguson admits Saints were made to pay for a lack of physicality as they were beaten 54-28 by Saracens at Franklin's Gardens on Sunday afternoon.
The home side lost the second half 31-7 after producing a promising first-half performance.
Saracens eventually ran riot, meaning Saints have now lost all three of the Premiership Rugby Cup games they have played this season.
And assistant coach Ferguson, who was in charge of the team this week as part of boss Chris Boyd's coaching rotation programme, said: "It was pretty much exactly what we spoke about in terms of the fact any time you play Saracens you have to front up to the physical battle and we lost that battle in too many areas.
"It's a pretty clear picture for me and one that can be fixed. If you're on the back foot from a physicality point of view, you're on the back foot for the game.
"We certainly gave them too much in the first 20 minutes but the boys did dig in really well.
"In the second half, we showed what we had done in the past two games and as I said previously, our challenge is to be consistent for a longer period of time.
"If you don't have the ball or you have the ball and don't look after it, it's going to bring defensive pressure.
"If we were more ruthless with our physicality consistently, we wouldn't have ended up defending as much.
"Physicality is different to effort, and I would never question these boys for their effort.
"But for me, too often, we were on the back foot physically, and rugby is a very hard game to play against anybody, never mind Saracens."