Bristol City manager Keith Millen issued an apology to supporters after their 1-0 Carling Cup defeat to Swindon.
A goal from Swindon's Raffaele De Vita proved enough to dump City out of the competition despite their opponents being two divisions below them.
Millen said: "That performance simply wasn't acceptable and I have a good deal of thinking to do."
He added: "We took the competition seriously by fielding a strong side but we were never in it and Swindon thoroughly deserved to go through.
"Our goalkeeper Dean Gerken picked up a groin injury in the first half, which is why David James took over for the second period. We'll have to see how Dean is before Saturday's trip to Doncaster.
"I also sent on winger Albert Adomah to try and freshen things up but it was a really bad night for us and I can only say sorry to the supporters."
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