Dale McIntosh said Cardiff Blues' defending was "woeful" in their 40-24 defeat by Treviso in the Pro 12.
Ninth-placed Blues conceded five tries as the Italian team recorded only their second win over the Welsh capital side.
Defence coach McIntosh would not talk about director of rugby Mark Hammett, who is expected to leave next week.
"I can't comment on that, but defence is my department and at times our defence was absolutely woeful," he said.
"So there's no one to blame but myself and obviously the players because it is my department."
Blues conceded two tries in the opening quarter of an hour against the side lying in 11th place in the table.
And Treviso had secured a bonus point before half time as full-back Jayden Hayward crossed for his team's fourth try after running through a huge gap in the Blues' midfield.
"It's about the players applying what we do in training," added the former Wales international.
"We do it all in training, we do the tactics we do the systems and I'm just stumped. I just don't know what happened out there today."
The Blues have won just four out of 15 matches in the league this season.