Christian Day feels Saints were their own worst enemy in Saturday's defeat to Ospreys.
The lock believes individual errors were the key to his team's undoing in their 43-32 loss, which was a ninth in 10 matches for Jim Mallinder's men.
Ospreys, who had not won away in Europe's top tier tournament since 2009, scored six tries at Franklin's Gardens.
Saints did fight back in the final 20 minutes, and they scored five tries of their own.
But it was too little, too late as the soul-searching continued at the Gardens.
"We need to look at ourselves," Day said. "There's no point looking for a quick fix.
"I missed a tackle for a try and everyone has got something to work on.
"We knew the Ospreys were a good team - they've got British Lions playing for them and they opened up a big lead.
"We were our own worst enemy and we're very disappointed.
"It wasn't a lack of effort or physicality - it was mistakes.
"I wasn't trying to miss a tackle on their full-back (Dan Evans), who had a great game, and he put seven points on the board.
"It was two teams wanting to win and we weren't good enough."