Walsall boss Jon Whitney said his side's performance in their 5-1 defeat by Rotherham on Tuesday left him "sick in the stomach".
The Saddlers were 4-0 down after 31 minutes at the New York Stadium, with all the goals conceded in an 18-minute spell in the first half.
"For 30 minutes we didn't compete and do the basics," Whitney, 46, told BBC WM 95.6.
"I apologise to the travelling supporters, we dropped well below par."
Although Amadou Bakoyoko pulled one back for Walsall early in the second half, Rotherham ran out easy winners as captain Lee Frecklington completed his hat-trick.
"Sometimes you've got to put your hand up and say you've been absolutely dire," Whitney said. "We'll have to reflect and have some honesty because that can't be allowed to happen.
"I feel sick in the stomach, angry and determined to put it right. We dropped so low beneath our standards - we didn't see that coming."
The defeat was the Saddlers' second in three days and third in four away games this season
Whitney knows results on the road are making their unbeaten home run important, adding: "They've put massive pressure on themselves now. It's a must-win game against Peterborough at the weekend.
"We'll look at it logically and we have to react."