Yann Kermorgant and Danny Green struck in the space of three minutes after an even opening half-hour.
United saw plenty of the ball, but failed to open up the visitors and Kermorgant’s free-kick on 58 minutes sealed a comfortable night for the Championship side.
With the exception of a bright display from Josh Ruffels, Wilder took no positives from the game and felt the U’s had fallen below the standards expected.
He said: “We’ve had a decent run in the FA Cup and earned a couple of quid.
- Michael Raynes and Deane Smalley challenge for a high ball
“We worked hard to get into the third round, but ultimately this is a disappointing night for everybody.
“We took a step back. I don’t mind going out of the cup, but you want to make sure we go out with a bit of a bang.
“We haven’t pushed the opposition hard enough and gone out of the back door, which is a touch disappointing.”
United have struggled in front of goal in recent weeks.
Their one effort on target came in the 78th minute and Wilder is keen to bring in more firepower to boost the side’s promotion chances.
- Deane Smalley sets off on a run
He said: “We’re working hard to get one or two in because I think we need to freshen it up.
“Maybe a couple of them need to come out of the team.
“If we do that, hopefully we can go again.
“I’m pushing and hopefully we can make a decision, because I think we need it.”
A deflated Ryan Clarke said: “We didn’t do ourselves any favours and we’re all disappointed.
“We know we have let ourselves down, all of us except Josh, who was outstanding.”
Ryan Williams missed the game with a back injury sustained in Saturday’s 1-0 win at Wycombe Wanderers.
The winger is expected to recover in time for Saturday’s home game against Torquay United in Sky Bet League Two.
- Post-match reaction from manager Chris Wilder and goalkeeper Ryan Clarke, via Oxford United's Youtube channel