Jim Mallinder admitted referee Andrew Jackson had no option but to send off Kieran Brookes in Saints’ defeat to Newcastle Falcons at Franklin’s Gardens.
Brookes was dismissed towards the end of the first half for a forearm charge on Falcons hooker Scott Lawson at a ruck.
Saints were 8-3 down at the time after a very poor start to the game, but they battled back and led 13-8 and 16-15 before a Vereniki Goneva try and Joel Hodgson conversion won it for the Falcons with nine minutes to go.
And when asked about Brookes’ dismissal, Mallinder said: “I’ve looked at it a few times and what happened was that someone put their head into the ruck, they came in and caused that first offence, Kieran Brookes took exception to that and their players took exception to Kieran Brookes.
“What you can’t do is retaliate. You can’t strike anybody near the head and the referee had no option.
“If you’re behind in the game or ahead in the game, you don’t want anyone getting sent off - it puts your team under a lot of pressure.
“But I think the 14 men responded really, really well in that second half and we should have come away with the win.”
Saints had been on a four-match winning streak going into the game.
But they will now head to Leicester Tigers next Saturday needing to bounce back in style.
Mallinder said: “We’re very disappointed. It was a poor start, we went behind and to lose Kieran Brookes just before half-time is what you don’t want to happen, down to 14 men without your tighthead.
“I was pleased with the way we responded, we showed some pride in that second half and I suppose we should really have won the game.
“We were doing what we needed to, getting into their half, driving the lineout, but unfortunately the intercept (for Goneva’s try) really cost us.”