Pablo Zabaleta admitted West Ham were well below-par during their 4-0 defeat at Oxford and said sorry to the travelling supporters.
The Hammers were knocked out of the Carabao Cup in the third round at Kassam Stadium with the U's scoring four times in the second half.
West Ham were backed by 1,513 away fans, although most had left by the time the brilliant Shandon Baptiste dribbled into the area and curled into the corner in stoppage time to make it 4-0.
It followed earlier efforts by Elliott Moore, Matty Taylor and Tariqe Fosu as Oxford reached the fourth round for the first time in 22 years.
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."
Oxford's stunning victory continued manager Karl Robinson's impressive record in this competition.
During his tenure as MK Dons boss, the 39-year-old knocked out Blackpool and Norwich when they were in the Premier League before recording a famous 4-0 win over Manchester United in the third round five years ago.
Robinson also triumphed over QPR – when they were in the top flight – in the FA Cup, but played down his cup exploits.
"I just like winning football matches, no matter who you play against. I think one thing my teams always do is show tremendous respect for the opposition, but we never change our style," Oxford's manager said.
"We almost in a stupid way match them at their own game and sometimes it doesn't work.
"I've been battered 6-0 in this competition by Southampton. I've also been beaten 5-1 by Chelsea in the FA Cup, so there have been a few hidings, but these are the ones you remember for the rest of your life.
"Before the game I said I think I have some Premier League youngsters in my team and if you look at our age, we were younger than West Ham.
"We don't only have an older group of players who want to run around, but we have a young team with tremendous growth thanks to the academy and the players we have brought in so I can't ask for any more from my players."