Pompey held the joint second-best defensive record in League Two last season.
But Alan Knight believes goalkeeping errors were still critical to their promotion aspirations.
And he remains concerned the long-running issue has not been resolved ahead of the Blues’ new campaign.
With four keepers used in five pre-season matches to date, Paul Cook continues to search for an answer.
Pompey have taken on three goalkeeping triallists this summer, while Jamie Ashdown trained for a week in an unsuccessful attempt to win a deal.
Yet, worryingly, the hunt continues to return empty-handed.
Blues legend Knight said: ‘We had the second-best defensive record in the league last season – and it is a very valid point.
‘But people will rightly point out the fact goalkeeping blunders at critical times in critical games did cost us.
‘Granted, there were not a lot let in, but they were important and it did affect the team. Look at Ryan Allsop’s two mistakes in the play-offs.
‘I don’t want to dig out goalkeepers, that is not what I like to do.
‘However, the problem at this level, playing for Pompey, is their concentration levels have got to be right and that has troubled some keepers.
‘Considering the amount of possession we have and the domination in the opponents’ final third, there will be long periods of inactivity for the keeper.
‘Yet we were getting caught out on breaks and set-pieces.
‘I am not teaching people to such eggs but the hallmark of a good keeper is to take control of a situation, to be commanding, to organise your defence.
‘The problem we have had is finding one of those.
‘Ryan Fulton has the makings of a good keeper and will do well at Chesterfield, but he is learning and will make mistakes.
‘You are caught between the devil and deep blue sea. Teams loan out a young keeper to get experience but that experience means them making mistakes – and that is the position we are in.
‘We are looking for an experienced goalie, but in theory had that with Brian Murphy, although he hadn’t played that many games in a season for quite some time.
‘We all know we need a new goalkeeper.’
Paul Jones could make his return from injury against Bournemouth tomorrow to provide another alternative.
And Knight believes the importance of a reliable keeper can be overlooked.
He added: ‘The goalkeeper is still a massively important position.
‘I am old school and still believe you build a team with a keeper, centre-half, midfielder and striker. I don’t see that ever changing.
‘Football has evolved massively since I played, the backpass rule killed me.
‘These days keepers have got to be so much more, distribution is one such aspect.
‘But sometimes people forget about the shot-stopping side of it.’