The popular striker was handed a start for Ian Foster’s side against Southend at Westleigh Park.
And he responded in typical fashion with a goal in each half as the Blues clinched a 2-1 victory over the League One club in an entertaining tussle.
Foster employed a 4-1-4-1 system, with Chaplin leading the line in front of watching Pompey boss Paul Cook in a crowd of 240.
Ben Close was named as skipper upon his return from a hamstring injury which had kept him out of Blues duty so far this season.
Gareth Evans lined up on the right-hand side of midfield, one of the three allotted over-23 year-olds along with Amine Linganzi and Adam Buxton.
Liam O’Brien started in goal for his maiden Pompey appearance at any level since leaving the club in 2010, while there was also an outing for Ben Tollitt.
When the match got underway, a fine block at the near post from Theo Widdrington to cut out Danny N’Guessan’s left-wing cross thwarted the lurking Robinson.
Yet on 13 minutes, the Blues broke the deadlock through Chaplin.
Adam May fed the youngster down the right-hand side of the penalty area and he unleashed a ferocious shot into the top corner of the net.
Moments later, Evans crossed from the right and Chaplin sent a header looping onto the roof of the net with the keeper struggling.
The diminutive striker was looking sharp and on 20 minutes May flighted a pass from deep into the box which was met with an angled left-foot half-volley which narrowly drifted wide of the far post.
May and Chaplin were demonstrating an excellent understanding and in Pompey’s next attack the forward sent a backheel into the path of May, whose right-foot shot was saved.
Southend came within a whisker of levelling on 30 minutes when O’Brien blocked and Joe Bedford sent a shot towards the empty net only for Widdrington to hack the ball off the line with a sliding clearance.
The hosts doubled their advantage on 58 minutes following excellent work from Tollitt.
The winger launched a rapid counter attack down the left before unselfishly squaring for Chaplin to finish under the body of Smith.
Southend pulled one back on 66 minutes when Harry Phillips’ free-kick delivered from the right was headed home at close range from Daniel Matsuzaka.
Chaplin thought he had his hat-trick on 77 minutes when he steered a header from May’s long pass over Smith, only for the keeper to somehow claw it away.
And it remained 2-1 to Pompey as they booked their place into the next round of the competition.
POMPEY: O’Brien, Buxton (Oxlade-Chamberlain 65min), Saidy (Bedford 77min), Widdrington, McDowell, Close, Evans, Linganzi, May, Tollitt, Chaplin.
Subs Not Used; Bass, Wakely, Bradbury.
SOUTHEND: Smith, Williams, Matsuzaka, Lang, Bedford, Batlokwa, Kyprianou, Phillips, N’Guessan, Robinson, Cotton (Wabo 65min).
Subs Not Used; Bexon, Walker, Bwomono, Clifford.