The team selection was bold, some had screamed folly, but Kenny Jackett’s experimental Pompey line-up claimed a second Championship scalp.
And with it his Blues side progressed into the third round of the League Cup for the first time since 2010.
With Christian Burgess at right-back and Tom Naylor in the centre of defence, understandably there were doubts outside the Pompey dressing room.
A pitiful late defensive performance against nine-man Coventry had deprived the Blues of two points in their previous outing.
So, for the subsequent trip to QPR, Jackett opted for a reshuffle which was not entirely embraced by those of Pompey persuasion.
The outcome, however, was a clean sheet - and victory at the Kiyan Prince Foundation Stadium.
The Blues secured a 2-0 triumph courtesy of two goals in the final 13 minutes, adding Rangers to the Birmingham on the roster of clubs eliminated from this season’s competition.
John Marquis had broken the deadlock from the penalty spot on 77 minutes, having won the decision himself.
Then Ross McCrorie squared for the ever-impressive Marcus Harness to seal matters on 81 minutes and book that third-round presence.
Granted, there was late drama when substitute Ebere Eze pulled one back in stoppage time, only for referee Ross Joyce to declare it offside following consultation with his linesman.
Yet the unfamiliar defensive line-up remained unbreached, accomplishing their instructions superbly at the home of Rangers.
Certainly Jackett would have earned criticism should his brave decision have backfired - that Burgess introduction off the bench against Coventry remains raw.
Nonetheless, he warrants credit for the Carabao Cup outcome as this erratic August comes to an excellent end - and potentially new defensive personnel to consider.
Jackett had made two changes to the side which drew 3-3 against Coventry, yet it was the positional selections which represented the biggest surprise,
The return of Burgess for the out-of-sorts Sean Raggett may have been anticipated by some, but not the role the long-serving defender was handed.
Burgess was instructed to line up at right-back, freeing McCrorie to play in his favoured central-midfield role.
Save a couple of substitute appearances in the position for the Blues, it represented the 27-year-old’s maiden outing with such a remit.
The accompanying reshuffle instead saw Naylor drop back to partner Paul Downing in the centre of defence, thereby creating space for McCrorie to play alongside Ben Close in midfield.
With Lee Brown sidelined through an Achilles injury, Brandon Haunstrup was the obvious choice to replace him at left-back.
Although Jackett is still assessing James Merredith in training as a potential signing to bolster his left-sided options.
Pompey’s bench included the omitted Raggett, while Anton Walkes was recalled to the squad having been excluded from the match-day 18 against Coventry.
Elsewhere, Haji Mnoga was not considered for a right-back role after injuring his ankle at the weekend, while James Bolton didn’t make the squad as he continues his recovery from an ankle problem.
When the match got underway, an uneventful opening sprang into life on 10 minutes following a marvellous QPR move, emanating from keeper Liam Kelly playing it out.
The ball was eventually played inside Naylor to Jan Mlakar, whose low cross from the right narrowly eluded the slide of Bright Osayi-Samuel at the far post.
Craig MacGillivray, who received his maiden Scotland call-up 24 hours earlier, was called into action for the first time on 22 minutes.
Clever work from Ilias Chair down the right saw him switch the ball to his left foot and deliver a shot from outside the box which Pompey’s keeper gathered comfortably around his head.
At the other end, McCrorie’s right-footed shot from 30 yards saw Kelly scrambling across his goal to keep out with a diving save to his left.
Ryan Manning collected the game’s first booking on 40 minutes following a late challenge on McCrorie after the Pompey man had delivered a cross from the byline down the right.
It was goalless at the break and 10 minutes into the second half Marc Pugh had a great chance to break the deadlock.
Marcus Harness’ clearance succeeded in only finding Manning down the left and his cross was met with a Pugh header which flew straight at MacGillivray.
The visitors were coming under increased pressure, challenged to defend their penalty area for a sustained period, but still the deadlock remained.
Jackett made his first substitution on 60 minutes, with skipper Evans replaced by Andy Cannon in an attacking central midfield role.
There was a rare Blues foray up the pitch of 67 minutes when a cross from the left fell invitingly into the path of Burgess, who surged forward and crashed a first-time 30-yard shot.
His spectacular attempt narrowly whistled over the bar, with the defender appealing in vain for a corner.
Substitute Nahki Wells, already booked for fouling McCrorie, was fortunate not to receive a second yellow card after blatantly obstructing Harness, earning a talking to from referee Ross Joyce.
On 76 minutes, Pompey earned themselves a penalty through Marquis.
McCrorie fed the ball into the feet of the striker, who cleverly rolled Toni Leistner inside the box and was caught by the QPR skipper.
Marquis then stepped up for spot-kick duty, sending Kelly the wrong way with a confident right-footed finish to present the visitors with the lead.
Barely four minutes later and the Blues had doubled their advantage, following excellent work from McCrorie.
Marquis released the Scot down the left and he surged past the flat-footed Leistner before having the presence of mind to square the ball across goal for the unmarked Harness to finish first-time right-footed.
As the match headed towards full time, Marquis had an opportunity to grab himself a second, but his low shot was well saved by Kelly.
The damage had already been inflicted by Pompey, though. Victory - and Carabao Cup progress - was their’s.