Aitor Karanka's side couldn't really have too many complaints about the result as they largely failed to shine in the attacking third of the pitch.BCFC
The only goal of the game was scored by U's centre-half Robbie Cundy who headed home an 18th minute free-kick.
Aitor Karanka's side couldn't really have too many complaints about the result as they largely failed to truly shine in the attacking third of the pitch, despite dominating possession.
However, credit must also go to League Two Cambridge who were spirited and well organised.
Summer signings, goalkeeper Andres Prieto and defender George Friend made their first starts. Meanwhile, midfielders Jon Toral and Adam Clayton, were named on the substitutes' bench and entered the fray after he break for their debuts.
It was also big day for 18-year-old forward Adan George who made his first professional appearance.
On a sunny September afternoon at the St. Andrew's Trillion Trophy Stadium both teams took a while to get into the flow of the game, with several passes going astray.
The first half-chance dropped to Cambridge's Luke Hannant after just seven minutes, but he was well off target with his strike on goal from outside the area.
Moments later Jeremie Bela was forced to undergo a prolonged period of treatment after falling on his shoulder, but fortunately he was able to return to the action several minutes later.
The U's were more than holding their own in the opening stages, with the hosts struggling to find a way through. With just 18 gone Cambridge took the lead from a free-kick.
Harlee Dean was penalised for a foul on Paul Mullin. Hannant launched a long ball in towards the back post and although Prieto attempted to reach it, he fell short and Cundy rose highest to nod home with the Blues 'keeper well off his line.
Bela was inches away from levelling the scores after a brilliant attempt. Dan Crowley laid the ball off to him and he whipped in a sweet left-footed effort which hit the bar and careered back off it.
Although Blues were having the greater possession as the half went on, they were failing to make the most of it.
Half-time: Blues 0 Cambridge United 1
Cambridge made a lively start after the break and Joe Ironside was allowed to have a free header on goal, but luckily for Blues he headed straight at Prieto.
Then for Blues, Bela attempted an audacious volley from the edge of the area, but it flew well wide.
With an hour gone, Karanka made his first change of the game when he sent on Toral for Crowley. Then less than 10 minutes later Clayton was sent into the fray in place of Maikel Kieftenbeld.
The duo soon started to make an impression and started to give Blues a greater cutting edge.
Blues won a corner on the right and Toral floated the ball into the box, but Kristian Pedersen narrowly headed it over the bar.
With time rapidly running out Blues were doing everything they could to find an equaliser but were largely being restricted to shooting from distance because of an organised Cambridge back four, who sat in for more or less the entire second-half.
With just five minutes of normal time remaining Odin Bailey was sent on in place of Ivan Sunjic as Blues tried to muster as grandstand finish.
But ultimately it proved fruitless as Blues lost their first competitive fixture of 2020/21.
Blues (4-2-3-1): Prieto, Colin, Pedersen, Friend, Kieftenbeld (Crowley, 68), Crowley (Toral, 60), Bela, Dean, Lakin, Sunjic (Bailey, 85), George.
Subs (not used): Jeacock, Roberts, Dacres-Cogley, Seddon.
Cambridge United (4-4-2): Mitov, Knoyle, Digby, Taylor, Hannant, O'Neil (May, 58), Mullin (Knibbs, 82), Dunk, Hoolahan (Iredale, 78), Cundy, Ironside.
Subs not used): Burton, Dallas, Davies, Knowles.
Goals: Cundy (18).
Referee: Leigh Doughty