Conor Chaplin waited for his opportunity – and didn’t he take it when it arrived.
Paul Cook had named 18 different players in his first two starting line-ups as Pompey manager.
For both fixtures, the 18-year-old from Worthing, however, remained on the bench, the go-to man when circumstances necessitated.
Against Dagenham & Redbridge the Academy graduate who has impressed Cook so much featured for six minutes in the canter to victory.
Last night the introduction arrived in the 71st minute – and within five minutes he had instigated a Capital One Cup shock.
Pompey had been leading Paul Clement’s Championship side since the 49th minute when Adam McGurk’s debut goal broke the deadlock.
The scoreline remained until the 74th minute when Jason Shackell headed home at the far post from a corner.
A mere two minutes later, though, substitute Enda Stevens delivered from the left and there was Chaplin to steer home a right-foot shot for what would prove the winner.
A maiden Fratton Park goal for the forward and the second of his Blues career – and how his team deserved it.
Cook’s men were outstanding from start to finish, full of guts, drive and positivity to render their visitors second best. This was no triumph on the counter-attack.
Adam Webster was a rock, Jayden Stockley tireless up front, skipper for the night Ben Davies dogged and then there was goal-hero Chaplin.
It’s hard to believe 11,573, of which 1,134 were away fans, could generate quite so much noise inside Fratton Park for the televised match but somehow they did.
Chants of ‘Paul Cook’s Blue & White Army’ rang out long after the final whistle as the Fratton faithful rejoiced for the second match running.
Deserved, so deserved. Pompey are winning matches and entertaining their fans. It has been too, too long.
In Pompey’s starting line-up only Davies, Matt Clarke, Gareth Evans and Kyle Bennett remained from the weekend victory over Dagenham & Redbridge.
As anticipated, elsewhere across the team it was widespread changes, with debuts for Brandon Haunstrup and McGurk.
Both were unused substitutes on Saturday, while in Haunstrup’s case the start reflected his growing stature at the club after emerging from the Academy in the summer.
The Waterlooville youngster replaced Stevens at left-back while McGurk, whose pre-season had been hampered by injury, lined up as one of the attacking midfield three.
Among the other seven changes were Paul Jones in goal in place of Brian Murphy and Webster coming in at centre half to partner Clarke.
Nigel Atangana and Michael Doyle relinquished their holding midfield roles to Danny Hollands and Ben Close, with Stockley replacing Matt Tubbs in attack.
As for Derby, they named former England international Darren Bent up front, ex-Southampton full-back Chris Baird as skipper and summer recruit from Aston Villa Andreas Weimann as an attacking midfielder.
Pompey started brightly, looking composed on the ball and seemingly unnerved by their Championship opposition.
Certainly they were eager to get forward and test the Rams rather than focus on playing it safe in the opening stages.
The Blues had their first attempt on goal in the 13th minute when Haunstrup’s left-wing cross was cleared only so far as McGurk.
The former Burton Albion man sent in a snap-shot which zipped through the penalty area, out of reach of Stockley and out for a throw-in, but nonetheless offered encouraging intent.
Moments later Bennett cut in from the left and fired a right-foot shot from 25 yards just wide of the post with Lee Grant scrambling across his goal.
It was all Pompey and Davies and Bennett combined cleverly down the left before the ball was played back to Evans, whose was too slow to react and Jeff Hendrick stepped in to clear.
The Rams had their best moment yet in the 31st minute when Craig Forsyth crossed from the left and Webster brilliantly threw himself in front of Weimann’s shot to deflect it for a corner.
It was developing into an intriguing contest and in the 41st minute Johnny Russell had plenty of space down the left to fizz in a cross which Clarke managed to hack off the line for a corner, despite Jones’ best attempts to gather.
Still it remained goalless at half-time, with Pompey receiving generous applause in recognition of a very good opening half.
With no substitutions at the break, Derby immediately came out stronger and within two minutes Weimann fired a shot from the right into the side netting. Yet it was Blues who took the lead on 49 minutes through a goalkeeping error from Grant.
Stockley crossed from the right and McGurk rose to plant a header which the flying Grant appeared to have covered only to allow it into the net.
It was an awful mistake from the Derby man, nonetheless Pompey had a deserved lead against their Championship opposition.
Fratton Park was rocking and on 56 minutes Stockley drove in a shot which was parried by Grant, then the keeper safely gathered Davies’ follow-up effort from outside the box.
The Rams made a double substitution on the hour mark with Bent and Simon Dawkins replaced by Tom Ince and Chris Martin.
Despite having the lead, Cook’s men were refusing to sit back and Evans drove in a low shot from distance which was comfortably handled by Grant.
However, in the 74th minute Derby had their equaliser when Russell’s right-wing corner was powerfully headed home at the far post.
Two minutes later, Pompey regained the lead.
Stevens pulled the ball back from the left and Chaplin steered a right-foot, first-time shot into the far corner to make it 2-1.
Derby were then denied a penalty when Hollands tripped Russell as he tricked his way into the penalty area, yet referee Paul Tierney ruled it was outside the box.
Russell took the free-kick himself, sending his effort comfortably over the bar.
And the hosts held on to deservedly booked their place in the second round of the Capital One Cup.