London - Northampton's director of rugby Jim Mallinder said he could not defend Dylan Hartley after the England captain was red-carded during his side's 37-10 European Champions Cup defeat by Leinster.
Hartley, 30, was dismissed just six minutes after coming on as a substitute at Franklin's Gardens on Friday for swinging an arm at Sean O'Brien, with England head coach Eddie Jones watching on.
He now runs the risk of an eight-week ban, jeopardising his chances of captaining England in the Six Nations and leading the British and Irish Lions in New Zealand next year.
"You can't strike to the head," Mallinder said. "We've seen enough of them over the last few weeks and I think it was a red card.
"Dylan has been playing really well and leading England by example, so it was disappointing on returning from England and doing such a good job that that happens."
Hartley skippered England to a record-equalling 14th consecutive Test win over Australia last week.
But the New Zealand-born hooker has a sorry disciplinary record, having been banned for a total of 54 weeks over the course of his career.
He was dropped by then England coach Stuart Lancaster - now on the coaching staff at Leinster - for the 2015 World Cup after headbutting Saracens' Jamie George.
But Jones, who succeeded Lancaster after the tournament, reinstated him, praising Hartley's "aggressive and uncompromising approach to playing rugby".
Television pictures showed Jones running a gloved hand down his face as Hartley was given his marching orders by French referee Jerome Garces in the 58th minute of Friday's match.
England open their Six Nations title defence against France at Twickenham on February 4.
Leinster coach Leo Cullen said Hartley's dismissal had "knocked the stuffing out of" Northampton.
While he did not defend Hartley's conduct, Saints captain Tom Wood suggested the hooker's demotion to the bench may have been a factor.
"I can only assume he is trying to make an impact and be positive," Wood said.
"I can't speak for him, but my impression is that he's on the bench, he's eager, it's a big European night and he wants to make an impact."
Former England centre Will Greenwood tweeted: "All sorts going wrong @SaintsRugby - @DylanHartley would not have done that in @EnglandRugby shirt - no excuse for it - but it tells a tale."
Hartley's dismissal, and the outcome of the match, completed a dismal week for Northampton.
They have been under fire after letting Wales wing George North play on against Leicester last week despite appearing to be concussed.