Brennan Dickenson made his much anticipated return from injury on Thursday evening, as the U's were beaten by Swansea City in the Premier League Cup.
The visitors led 2-0 and 3-1 before the U's got it back to 3-3, but their top flight opponents eventually ran out 4-3 winners.
It means that the U's cannot now progress to the knockout stages, with two more group games to go.
The Swans flew out of the blocks on Tuesday and forewent two good chances before they eventually took the lead on 14 minutes.
Courtney Baker-Richardson had been denied by a good save from Dillon Barnes, but powered home a header from a corner to give the visitors the advantage.
One became two six minutes later as Liam Cullen slid Baker-Richardson in on goal and he made no mistake as he put the ball past Barnes.
The U's responded well, though, and started to get a foothold in the game in the remainder of the half.
Will Wright was denied by a good save from Lewis Thomas, who tipped his header over the bar, before the U's did pull one back five minutes before the break.
A scramble in the Swansea area saw the ball ricochet across the six yard area before Decarrey Sheriff got the final touch to reduce the arrears.
The U's started the second half in positive fashion, but saw the deficit increase to two ten minutes after the restart.
Baker-Richardson and Rooney both went for the ball and although the Swans striker claimed for his hat-trick, it appeared that it went off the U's man last.
But the U's recent signing got on the scoresheet at the right end five minutes later.
He rose highest at a set piece to power home a header and make it 3-2.
Brennan Dickenson was introduced not long after and saw plenty of the ball after his entrance onto the pitch, with some early signs of his trademark runs forward.
With eleven minutes to go, the U's drew level in some style, as Sam Warde hit a first time half volley from 30 yards to drill the ball into the bottom corner.
But parity was to last just sixty seconds as Kenji Gorre's effort beat Barnes to make it 4-3.
The win keeps alive Swansea's hopes of progressing to the later stages, but the U's can now only finish third in the group at best.