A late goal denied the U's at least a creditable draw on Tuesday night, as Charlton ran out 2-1 winners at the JobServe Community Stadium.
The U's had taken the lead via a stunning strike from Luke Norris and had more than matched up their Championship opponents for much of the game.
But the visitors turned the scoreline around with two crosses from Alfie Doughty setting up an equaliser from Abraham Odoh and an own goal from Tom Eastman.
The visitors started the stronger, seeing plenty of the ball in the early stages without truly testing Ethan Ross.
They did have the first chance of note, though, as Williams laid it into the path of former U's man Harriott, who forced a fine low stop out of Ross who turned it behind.
A minute later and the U's had a chance of their own, as Hasanally drove forward out of midfield and lay it into the path of Nouble, who pulled his effort beyond the far post.
Norris was the next to go close. He stepped up to take a free kick that took a big deflection and Amos changed direction to tip the ball beyond his left hand post.
Charlton had the best chance to date when a ball from Harriott landed at the feet of Taylor but, on the swivel, he lifted his effort over the bar.
Cowan-Hall was the provider as the U's went forward just before the half hour mark, and his tempting cross was headed clear by Sarr with Norris set to head home.
Cowan-Hall sent over another cross on 34 minutes which eluded several players in the middle, with it eventually falling to the U's triallist and it was a brave block by Lapslie to see the shot away.
Charlton were the next go to close, with a clever turn by Williams in the box affording him the space to get his shot away, and it was a strong parry from Ross to clear it from danger.
Ross was having an impressive half and denied Morgan five minutes before half time, as the Charlton man broke into the box after a defensive lapse by the U's.
All square at the break, the U's took the lead four minutes after the interval.
Pell retrieved the ball in midfield and set up Norris, who let fly with a fantastic strike into the top corner.
Quitirna almost drew Charlton level, but Gerken was down sharply to save at his near post.
The U's summer signing was the home side's first sub of the game and he was followed by Courtney Senior ten minutes later, as he replaced Cowan-Hall.
Harriott tried to draw the visitors level, but his bending effort on 57 minutes dipped just over the bar.
The U's made further changes with the introduction of Vincent-Young and Brown for Hasanally and Kensdale.
And Senior almost doubled the home side's lead as he drove forward before smacking a shot just wide.
The Addicks should have been level midway through the second half, though, as the ball was set back to Quitirna in a great position in the middle, but he lifted his effort well over the bar.
But they did get their equaliser two minutes later as a great ball found Doughty down the left and his low centre was turned home at close range by Odoh.
The U's made seven changes with fifteen remaining, with Eastman, James, Lapslie, Chilvers, Stevenson, Gambin and Fernandes introduced into the fray.
Fernandes was straight into the action and almost scored as Senior put him in, but his effort was well saved by Amos.
But it was the visitors who took the win, as another dangerous Doughty cross deflected in off Eastman and into the far bottom corner.
U's: Ross (Gerken), Prosser (Eastman 75), Kensdale (James 75), Sowunmi (Chilvers 75), Cowan-Hall (Senior 55), Pell (Stevenson 75), Comley (Lapslie 75), Triallist (Fernandes 75), Hasanally (Gambin 75), Nouble (Vincent-Young 75), Norris (Brown 75)