Steve Cotterill has admitted Pompey lack the 'chemistry' to overcome their defensive vulnerabilities.
The Blues have leaked three goals in each of their past four matches.
It is a statistic made even more worrying with a trip to promotion-chasing Forest tomorrow and the likes of high-fliers Leeds and QPR on the horizon over the next two weeks.
Aaron Mokoena, Ibrahima Sonko, Ricardo Rocha, Carl Dickinson and Joel Ward have all been used in the back line in recent weeks.
Meanwhile, Hermann Hreidarsson has started just one match, although appeared several times as a substitute.
Whoever has been used, though, has failed to plug the gaping holes in Pompey's shaky defence.
Cotterill said: 'There's not really a lot extra we can do on the training ground.
'I think we are where we are as regards that.
'There's nothing wrong with an organisation point of view in how I set up my back fours. In the past my teams have had a record of keeping clean sheets.
'This team, unfortunately, isn't geared up to keeping clean sheets, so what we have been trying to do is outscore the opposition.
'For all the things to change, I think there should be a personnel change because we haven't quite got that chemistry right.
'When you come into this job and there is only one defender at your club and every player you try to get is a loan player to shore things up, sometimes you can't quite get the chemistry right.
'I haven't been able to piece that jigsaw right.
'Unfortunately, I haven't been able to do that. It has been an impossibility for me.
'But, to be honest, in the predicament we were in, we were lucky to get four defenders, let alone get anyone who is going to piece everything right.'
Talking about Hayden Mullins, Cotterill added: 'To have one player who has played in defence for your football club but is a midfielder – and has played midfield all his life – it is unique, really.
'The loan players who have come in have done their best and really been part of it.
'Sometimes we have kept clean sheets and sometimes they have been great.
'Other times we have had an individual error which has cost us.
'It's not just been them. There have been mistakes by all of them.
'Whoever has played, we have ended up with a mistake or two or three or four but what I would say is we need to defend from the front of the team as well.
'It is not just down to them and the goalkeeper – we attack as a team and we defend as a team.'
'That part is important for them as well.