Rafa Benitez defended his decision to make nine changes after Newcastle United crashed out of the FA Cup.
Oxford United beat Benitez's weakened team 3-0 at the Kassam Stadium this afternoon to reach the fifth round of the competition.
Aleksandar Mitrovic had two first-half shots saved and was also denied from the penalty spot after the break.
All three Oxford goals came in the second half, when Newcastle – who had ended the first half strongly – fell apart.
Benitez rested a number of players ahead of Championship home games against Queens Park Rangers and Derby County.
And United's manager defended his starting XI after the match.
“We didn’t underestimate Oxford," said Benitez. "We didn’t take our chances in the first half, then we missed a penalty, and they took the chances that came their way.
"We didn’t mark properly from the corners, and we gave the ball away. You have to give credit to them, because they were fighting for the result, but we made too many mistakes.
“We had to make the changes. It is very clear. I said before the game that if we were going to go through, it had to be with the squad, and if we couldn’t do that, then we couldn’t.
"We could have been ahead in the first half, but we didn’t do it and then we paid for the first mistake in the second half and that was it."
Oxford manager Michael Appleton said: “We’re delighted we’re in the last 16.
"It’s a fantastic achievement for the club. We thought it was on when the second goal went in, but it was the save from the penalty that gave the players more belief than actually going ahead in the game.
"My celebration was certainly bigger. It gave the players a massive belief that it might be their day."