Alan Knill apologised for Rotherham's first-half display after League One's bottom club lost 3-0 at Northampton - leaving them 13 points from safety.
Knill's side are worryingly adrift at the bottom of League One
"We just can't keep conceding goals like that," the Millers boss said after they had trailed 3-0 at the interval.
"We are missing some quality in the team - we lost that when we sold Will Hoskins and Lee Williamson (to Watford in January). It's now taking its toll.
"Changing formation stemmed the tide, but gave us nothing going forward."