A furious Chris Wilder kept his Oxford players in the dressing room for 45 minutes following the 3-0 defeat at Port Vale on Monday.
The U's boss admitted the second half was "like men against boys" as his side crashed to a fifth away defeat in six games.
Wilder said: "For the first 25 minutes we were well in the game. We got in some great situations where just the final ball was lacking and we really should have gone ahead.
"But as soon as the first goal went in we started to lack belief and then we've conceded a really poor second goal early in the second-half.
"After that it was comfortable for a Vale team who are in form because our heads went down and we lost contact with the game.
"Our players started to look after themselves and we really need some men in the side. It was a little bit too easy for Vale and that is a concern.
"There are no excuses and we've taken a step backwards after what had been a decent 10 days for us."