The U's have been beaten 2-0 at Peterborough United to ensure that it's an important few weeks coming up.
The home side went ahead on twenty five minutes, with skipper Tommy Rowe heading in a Grant McCann corner.
Assombalonga doubled the lead in the second half, lashing his low shot across Sam Walker into the bottom corner.
Boss Joe Dunne made three changes to his starting line up, bringing in Tosin Olufemi, Craig Eastmond and Clinton Morrison.
Olufemi came in at right back to replace the suspended Brian Wilson and Jabo Ibehre dropped down to the bench to accommodate Morrison.
Marcus Bean travelled but was sent home after feeling ill prior to the game, giving Eastmond a starting berth in the U’s midfield three.
Both managers wanted their respective sides to get forward as often as possible but it was the home side who had the better of things in the early stages.
They didn’t test Sam Walker in the U’s goal but Tom Eastman had to block sharply to prevent Assombalonga and a Grant McCann curler rebounded off the inside of the post to safety.
Inbetween those two chances, Freddie Sears and Clinton Morrison conbined and the latter’s cross found Vose at the back post.
Unfortunately, Dominic Vose could only hook his effort over the bar at full stretch.
On twenty five minutes, the U’s went behind, with McCann providing the ammunition from a corner.
His set piece whipped into the far post and Posh skipper Tommy Rowe rose highest to put his header past Walker and into the back of the net.
With half an hour gone, Sears found half a yard for himself but his effort was blocked and deflected for a corner, which came to nothing.
In a half that neither team really tested their opposing keeper, Assombalonga had his effort blocked and the home side also turned away from another few chances to shoot from the edge of the box.
The half time whistle went with the U’s a goal down and boss Joe Dunne would have been imploring his side to push further forward as much as they could.
That push forward didn’t materialise though, and it was the hosts who were on the front foot in the early stages of the second half.
Sam Walker made a great save with his foot from Swanson, which the referee missed and gave the U’s a goal kick.
David Wright’s speculative volley from outside the box flew over and Craig Eastmond was unlucky not to get in when he almost pounced on a defensive mistake but the U’s weren’t testing the Posh keeper.
That came back to haunt them on sixty four minutes, as Peterborough doubled their lead.
Magnus Okuonghae tried to thread a ball out to Gavin Massey, on as a sub for Vose, and Swanson got the block in.
The loose ball fell to Assombalonga and he made no mistake as he broke towards goal and scored with a drilled shot across Walker and into the bottom corner.
Two more attacking subs, Ibehre and Drey Wright failed to galvanise the U’s and it nearly got worse in the final stages.
Assombalonga shot wide from the angle and Walker saved well at the near post, before the referee blew the whistle for full time.
The U’s now have six matches remaining, starting with the game against Tranmere at the Weston Homes Community Stadium at the weekend.