Wednesday were unable to build on their Sheffield derby win earlier in the week, falling to a 2-0 loss after second half goals from Caolan Boyd-Munce and Odin Bailey at Hillsborough.
Holdsworth, who is assisting academy manager Steven Haslam, told The Star: "It was a disappointing performance. Quite a few of them weren't at the races and it showed against a good Birmingham side.
"We created a few good chances in the first half which we didn't capitalise on and we fell away in the second.
"Their first goal was a great goal and the second was a penalty but there are a few of the boys off it at the minute."
The half was fairly even and the hosts should have taken the lead when Conor Grant beat the offside trap and latched on Fraser Preston's neat pass but the Irishman's composure deserted him when it mattered most as he fired tamely at Moha Ramos.
"When you have a good spell and are on top, you have to take your chances when they come along," said a rueful Holdsworth.
Despite their below-par showing, Holdsworth said there were some positives.
He said: "It was good to have Josh Dawodu back from injury and he came on for the last 20 minutes. The first-team lads, Ash Baker, David Bates and Jordan Thorniley, were steady and got more minutes under their belts and I thought Osaze Urhoghide was excellent at left-back.”