The young striker returned to the starting XI and grabbed two goals to hand the Blues a 2-1 success over Leyton Orient.
The 19-year-old also missed a penalty as Paul Cook went with two strikers up front against the O’s.
Pompey should have won by more as they wasted as stack of chances in front of a crowd of 16,564.
But the win moves them within two points of the top three as they get back to winning ways.
There was a touching minute’s applause for former Blues and Orient defender Paul Went and ex-England manager Graham Taylor before kick off.
And Pompey fans were given their wish as Chaplin partnered Michael Smith up front in a 4-4-2 formation.
The home side produced incessant pressure against the strugglers from the outset.
Kal Naismith’s free-kick was gathered after six minutes and keeper Alex Cisak was forced to tip over Tom Parkes’ sliced clearance 10 minutes later.
Naismith then wasted a glorious chance 10 minutes later, after being teed up by Smith 10 yards out.
The Blues should have taken the lead after 22 minutes when Enda Stevens was brought down by Sammy Moore inside the box, but Chaplin’s spot-kick was the perfect height for Cisak to push around the post.
Chaplin atoned for his miss seconds later, though, when he poked home Carl Baker’s cross at the back post.
The Blues pressure continued but Smith’s header was cleared off the line by Parkes.
Naismith then slotted a shot wide of the post with the goal at his mercy.
And Pompey paid for their wasted chances when Gavin Massey produced a lovely finish from the edge of the box eight minutes before the break.
It was Orient’s only effort but underlined the cost of not putting the game to bed early on.
The introduction of new boy Jamal Lowe for Gareth Evans at the break gave the home crowd a lift.
And they were cheering two minutes after the restart when Chaplin put his side back in front as he glanced home Baker’s cross.
The Blues were still controlling the game but had a huge let-off after 65 minutes when Christian Burgess and David Forde got in a mix, but the defender’s header past the keeper went out for a corner.
Smith could’t quite reach Chaplin’s drive across goal with 10 minutes left and then Steven’s thunderous shot across goal went out for a corner.
But Pompey held out for a win which leaves them two points off the top three.