Northampton captain Dylan Hartley admitted his side's display against Ulster "was not good enough" after slipping to a 25-6 defeat at Franklin's Gardens.
The Aviva Premiership team became the latest opponent to be beaten by Ulster, who scored four tries to make it 13 successive wins this season.
Unless the Midlanders do something remarkable back at Belfast's Ravenhill next weekend, their European campaign will be over.
"That was not good enough from us," said England international Hartley.
"We speak about intensity, they brought bags of it and we didn't bring enough. They are unbeaten and they showed why. They are a good team but we made them look better than it could have been.
"We had internationals returning, injured players returning and our strongest team on the field but you can't just turn up and expect it to happen and I think that happened tonight. We got showed up by a good team.
"They did exactly what we would like to do to another team. They forced us into errors. It is not good enough."
Hartley's misery could increase if he finds himself in hot water with the disciplinary officials after an off-the-ball clash with Rory Best.
"I don't think it was an incident," the hooker added.