Up until the 4-0 home hammering by Hull on January 23, the Blues possessed the most miserly defence in the league.
In the subsequent six weeks, they have shipped 19 goals in 10 games – including Saturday’s appalling 4-1 loss at struggling Northampton.
Now seven teams have a better defensive return than Pompey.
Jackett told The News: ‘When you give goals away like we did (against Northampton), it’s very difficult to talk about the forward play because you need to have that base to go off.
‘A free-kick, two free headers from corners from three-yards out, and a ball over the top where we were in control. That made it frustrating for us.
‘With that fourth goal, we presented Sam Hoskins with the opportunity, he has sidestepped one of the defenders with a chest pass, which he has taken into his path, and chipped it over the keeper.
‘Very disappointing goals against us and ones that are avoidable. Unless you can get a good base, it’s very hard to then get a foothold in the game.
‘With our recent defensive record, it’s always a combination of things, we can’t say it’s down to one player or one unit necessary, you score and concede as a team.
‘As a side, we didn’t get it right and our defensive display, particularly in the early part of the game, wasn’t what we needed to compete and get points.
‘We have made mistakes. If you want to earn confidence again you need to roll your sleeves up and make sure you go again, that’s where the players are.
‘It’s a disappointing result and certainly in the first 30-40 minutes the game was out of sight for us.’
Among the chief culprits was Rasmus Nicolaisen, who had a wretched game on his fifth successive start.
He was among four players substituted on 59 minutes, with James Bolton replacing him and Jack Whatmough moving over to centre-half.
Jackett added: ‘Rasmus is learning and, in terms of his competitiveness, he has to keep working hard and adjusting to our football.
‘He is good enough to do that, he’s had some very good games.
‘Rasmus has had other games where he has been fine, there have been no problems.’
A message from the Editor, Mark Waldron
On Saturday, March 13 Pompey will finally get their day at Wembley.
To celebrate the Blues’ place in the 2020 EFL Trophy Final, The News has launched an offer that gets you 25% off all our Sports subscriptions.
You can choose your perfect Sports subscription here and use the discount code ROADTOWEMBLEY25 to get the latest news from Fratton Park for less. Offer runs until midnight on March 15.