The U's lost 4-1 at Millwall on a tough afternoon in London for Tony Humes' men.
First half goals from Lee Gregory and Shane Ferguson gave the hosts a two goal lead at the break.
Half time sub Chris Porter pulled one back, but Millwall added further goals through Ferguson and Byron Webster.
Boss Humes made three changes to his starting eleven, two of which were enforced.
Matt Briggs and Frankie Kent were too ill to take their places in the U’s defence, with George Elokobi and Tosin Olufemi coming in.
The other change came further up the pitch, with Gavin Massey coming in for Tom Lapslie.
Although they were the away team, the U’s were dominating possession and getting into the opposition half regularly.
They didn’t test the home keeper in the early stages though, the nearest they came was a deflected shot from Marvin Sordell in the third minute.
In what was the first real attack at the other end, the hosts opened the scoring through Lee Gregory on twelve minutes.
Tom Eastman misjudged a clearance, allowing the Lions’ forward to run through and clip his shot past Jamie Jones in the U’s goal.
It was a blow after the start the U’s had and certainly gave Millwall the confidence to get forward much more.
Steve Ferguson dribbled through two challenges and put a cross just too long, before Byron Webster headed wide from a corner.
On twenty eight minutes, it should have been two but for a poor finish from Gregory.
George Elokobi’s header back to Jones looked short and Gregory nicked it past the U’s keeper but hit the post as he rolled the ball goalwards.
Geoge Moncur curled wide as the U’s tried to get back into the game, and Olufemi got forward to shoot straight at Jordan Archer.
On thirty seven minutes, another defensive error gave Millwall a second goal.
Elokobi mis-kicked under no pressure on the edge, allowing Morison to roll the ball to Ferguson who shot through Jones’ legs to make it 2-0.
With four minutes to go until half time, Jones kept the lead to two, as he dived full length to tip a Morison header wide.
Tony Humes made a change at the break, replacing Marvin Sordell with his skipper Chris Porter.
And it took the former Sheffield United forward only nine minutes to reduce the arrears, drilling home into the bottom corner form a Harriott cross.
The goal could have given the U’s the impetus, but it was the hosts that reacted better.
O’Brien blazed a shot over the bar on fifty five minutes, when he had one or two teammates in better positions, and a third Millwall goal came six minutes later.
Alex Gilbey lost possession in midfield, Morison and Gregory combined before the latter played in Ferguson on the left.
He steadied himself before firing his side 3-1 ahead, giving the U’s a mountain to climb with less than half an hour to go.
Harriott, who was trying to get his side going, shot wide after getting a yard of space for himself.
But the hosts finished the stronger, with Jones dropping on the ball after Elokobi blocked a Saville shot.
And three goals became four on eighty minutes, as Byron Webster slammed home a rebound after Jones had saved a Morison header.
Millwall saw the game out comfortably, and the whistle went after four minutes of time added on with the U’s suffering a 4-1 defeat.
Defensive mistakes had cost the U’s but the reaction will have to come quickly though, with Crewe visiting north Essex on Tuesday night.