Arsenal 0 Middlesbrough 1
Having been soundly beaten twice by Arsenal's first team already this season, Juninho ensured that Middlesbrough exacted a measure of revenge against Arsene Wenger's reserves at Highbury.
The Brazilian, who also struck the post, was the clear difference between the two sides as men took on veterans and boys and managed to prevail.
Wenger may now need to turn to some of his rested stars - including Thierry Henry, Robert Pires, Fredrik Ljungberg, Patrick Vieira and Sol Campbell - if he wishes to reach the final.
Arsenal were at least able to field some experienced campaigners like Martin Keown, Kanu, Ray Parlour and Gilberto Silva.
However, they also had youngsters Quincy Owusu-Abeyie, David Bentley, Gael Clichy and Graham Stack in action at Highbury and, while they will benefit from the experience, their rawness still showed.
Danny Mills, meanwhile, was booed throughout by the Highbury crowd after his self-defeating attempts to rile Henry during the Premiership fixture at Highbury just under a fortnight ago.
At least he did not have to cope with Henry again, but Arsenal still threatened first down the other flank as Kolo Toure cut inside before unleashing a shot that flew just past the far post.
While Bentley also chipped just over the bar, it was Boro who came within a whisker of taking the lead on 16 minutes.
Juninho exchanged passes with George Boateng in a swift break before driving his shot against the inside of the far post and back into the arms of grateful goalkeeper Stack.
That spelt the start of a spell of Boro pressure, with Massimo Maccarone's fierce shot deflected narrowly wide and Juninho drifting dangerously into space.
However, Parlour, Gilberto and Edu managed to hold Arsenal's midfield together.
When a clear chance did arise, with Kanu cutting the ball back to Bentley in space, the 19-year-old blazed his first-time shot wildly over the bar.
Kanu's finishing was no better as he dribbled to the edge of the penalty area before hitting the same spot in the crowd after the ball had bobbled at the vital moment.
What was needed was calm, composure and class. And that was finally provided after the restart by Juninho.
Franck Queudrue, who had scored an own goal at Highbury 10 days earlier, muscled Martin Keown aside and when Gaizka Mendieta set up Juninho, he picked his spot inside the far post.
That at least spurred Arsenal into action and, with Mendieta off the pitch to receive treatment, Owusu-Abeyie shot wide, while Kanu just failed to pick out Bentley from the byline.
With Mendieta back, Boro nevertheless rallied and Maccarone's shot was parried at full stretch by Stack.
Still the half chances came and went for Arsenal as Keown headed over the top, Edu flashed a shot wide and Bentley's volley was saved by Mark Schwarzer.
Boro survived appeals for handball against Juninho as Parlour's cross struck the Brazilian, but they should have finished off with aplomb only for Joseph-Desire Job to miss an open goal.
Stuart Parnaby's fine cross found Job unmarked just a few yards out but he still managed to shoot over the bar.
Boro can only hope that miss does not prove too costly - especially if some of Arsenal's big guns return next week.