Phil Parkinson said he was left "shellshocked" by the "unacceptable" goals Wrexham conceded as they slumped to a 2-1 Championship defeat at home to Birmingham.
Wrexham were second best on a night when Birmingham could have scored several more goals, with two headers from Phil Neumann ending the visitors' winless start to the campaign and extending Wrexham's in the process.
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Neumann nodded in his first in the 13th minute, out-muscling the Wrexham defence to get on the end of Alex Cochrane's cross.
Although Callum Doyle levelled during a bright start to the second half from the hosts, Birmingham were back in front just after the hour when Neumann got in front of Zak Vyner to head in a second.
"In all of my time here, I think those two are the softest goals we've conceded," Parkinson said. "We've got to give ourselves a chance and defend the box better than we've done tonight. The first one, two of their players could have scored and that's unacceptable.
"In the second half we got the momentum we were looking for, we played with more conviction and personality, got back into the game and then conceded off a second phase, a poor clearance, it's unacceptable."
Parkinson sounded bewildered by the performance, and asked if it had left him shellshocked, he replied: "I'm shellshocked by the goals."
Wrexham dominated possession but barely created a chance in the first half on a night when Birmingham should have been out of sight by the break, with August Priske and Jay Stansfield missing golden chances.
"In the first half we didn't play with enough conviction, belief in the final third," Parkinson added. "We looked like a team of players who were waiting for somebody else to do something rather than taking responsibility.
"We addressed that at halftime and I thought we came out really well after the break, looked like the team we wanted to be, but you just can't concede goals like that and expect to win a game of football."
After narrowly missing out on a spot in the promotion playoffs last season, Wrexham now have just two points from three games to start the new campaign.
Parkinson's side, who are chasing a spot in the Premier League via what would be a fourth promotion under owners Ryan Reynolds and Rob Mac, began the campaign with 1-1 draws against Cardiff City and Watford.
They will return to action away at Millwall on Wednesday.
PA contributed to this report.