Kenny Jackett wants his side to shore up at the back as they chase play-off promotion to the Championship.
A two-legged clash with Sunderland awaits following Pompey’s final position of fourth in the League One table.
Ben Close cancelled out Sean McConville’s opener as the regular season ended with a 1-1 draw against Accrington.
Jackett made five changes for the Fratton Park contest in order to give some members of his squad a breather, with top scorer Jamal Lowe among those to make way.
The Blues boss said: “We’ve lacked energy at times in recent weeks and it was an emotional night on Tuesday, so we needed to freshen things up.
“Jamal looked very fatigued and he’s a big player for us, so this little break will do him good ahead of next weekend.
"The lines have blurred a bit in recent weeks and we’ve become too open at the back, without delivering through our front players."
“The play-offs are a big opportunity for us and I’m looking forward to it. It’s a cup competition now and different from the league format.
“We have to make sure we keep clean sheets because we haven’t had enough of them at home all season, which is up to the back four and keeper to organise.
“The lines have blurred a bit in recent weeks and we’ve become too open at the back, without delivering through our front players.”
Close also struck the woodwork for Pompey, as Charlton snatched third position on goal difference following a 4-0 rout of Rochdale.
Jackett said: “There were a couple of big chances in the first half, with headers from McConville and Burgess.
“But we picked up a head of steam after the break and Ben’s effort against the crossbar was excellent, while a couple other balls didn’t quite fall for Brett.
“Ronan put in some great deliveries and it was a lively performance from Solomon-Otabor on the left, so there were a lot of positives.
“We couldn’t quite get the flow that we wanted throughout the game, but it’s 88 points for the season and we can’t really complain about that.
“It’s a very high standard and now we have to focus on the play-offs, which are always unpredictable games.”