Zabaleta admitted West Ham were well below-par during their 4-0 defeat at Oxford and said sorry to the travelling supporters.
Stand-in captain Zabaleta told the official club website: "If you look at the performance, it's not been anywhere near our standard and also in the way we lost was not good.
"We are not the first Premier League team to be knocked out of the Carabao Cup. We saw Spurs (at Colchester) and we saw some other Premier League teams struggling to win games like that.
"Obviously we just need to move on now and look at the mistakes we have made and think about Saturday (away to Bournemouth).
"We return to the Premier League again and we don't have too much time to recover. It's only another 48 hours and we have another game and mentally we need to be strong for Saturday.
"We feel sorry about the performance and sorry to the away fans who came to the game because obviously we don't want our fans to see this kind of performance. It was just a bad night."