Robbie Cundy scored against his former club as 10-man Barnsley won 3-0 at Cambridge.
James Brophy hit a post for the hosts in the seventh minute, and it was a miss the U’s would regret.
Devante Cole opened the scoring in the 24th minute when he was played in by Liam Kitching’s pass and fired low into the bottom corner.
Goalkeeper Brad Collins made a fine save to deny Liam O’Neil shortly after half-time to preserve his side’s lead, but he was then taken off injured after a serious clash that contributed to 14 minutes of stoppage time.
When play restarted, Kitching brought down Harvey Knibbs and was dismissed after 67 minutes for a second booking.
Despite the disadvantage, former Cambridge loanee Cundy scored with a looping header to double the advantage nine minutes later.
Sam Smith and Jack Lankester had the ball in the net for Cambridge before seeing them disallowed for offside, then Barnsley substitute James Norwood ran clear at the start of stoppage time to bury the visitors’ third.