Manuel Pellegrini took full responsibility after West Ham crashed out of the Carabao Cup following a 4-0 hammering at Oxford.
The Sky Bet League One side scored four unanswered second-half goals via Elliott Moore, Matty Taylor, Tariqe Fosu and the excellent Shandon Baptiste to progress into the fourth round for the first time in 22 years.
It was a terrible result for West Ham, who made nine changes after the 2-0 win at home to Manchester United in the Premier League on Sunday.
"The whole team didn't play well. Not only did we concede four goals, but we didn't create too many chances," Pellegrini admitted.
"We missed too many passes from the beginning and the responsibility is first on me because I picked the players and second of course on the team that didn't compete.
"We talked enough with all of them to understand that the competition was different, the team was different and to play now against Oxford, who were in a good moment after winning 6-0 (at Lincoln) in their last game.
"The players must understand, but I repeat we couldn't do it because in front was a team that played very well. I don't want to take anything away from Oxford because they played very well."
Only Issa Diop and Pablo Fornals retained their starting berths from the win over Manchester United, but Pellegrini introduced Felipe Anderson and Sebastien Haller off the bench early in the second half.
He defended his team selection at the Kassam Stadium, insisting it was a necessity because of the fixture schedule West Ham have.
"I think in England if you want to play in all the competitions you need to use your whole squad. You cannot just continue with the same players that play on Saturday," Pellegrini added.
Oxford manager Karl Robinson was delighted with his players and urged the 9,000 home supporters to turn up again for Saturday's league fixture with Gillingham.
"We had the belief in ourselves that we could get a result," he added. "I'm over the moon for the fans and the players.
"It's about the players, about the fans and about the community of Oxfordshire and the big thing for me now is the people who came here for the first time in a long time, that they buy a ticket and come on Saturday.
"We want them to support us moving forward because economical growth is important for what we are trying to achieve."
Robinson, who masterminded a 4-0 win over Man United five years ago as MK Dons manager, admitted the result was one of Oxford's best at home in recent years, exceeding their FA Cup wins over Newcastle and Swansea.
"Looking at the scoreboard, 4-0 against West Ham and a very strong West Ham, this will probably go down as one of the biggest results at the Kassam in recent years," Robinson said.
Oxford will host League One rivals Sunderland in the fourth round and they will compete at that stage for the first time since 1997.