Martin Guptill says some poor results early in the CB40 competition look like costing Derbyshire a semi-final place.
Guptill's side moved up to third in their group thanks to a six-wicket win over Yorkshire on Sunday, with the batsman smashing an unbeaten century.
But some below-par displays - notably against the Netherlands - means they are only an outside bet to make the top two and qualify.
"We kind of shot ourselves in the foot really," Guptill told BBC Radio Derby.
Derbyshire are one point behind second-placed Middlesex, and have also played a game more than them.
The top side in each of the three groups progress, as do the best-placed runners-up.
But they only have two matches left in the competition and, even if they did overtake Middlesex, would have to rely on other results to clinch the best second-placed finish.
The New Zealander added: "We have a couple of close losses we should have won but we have to take it on the chin and play the best cricket we can for the rest of the games."
Derbyshire are next in action on Wednesday when they take on Northants in the County Championship.