According to Mark Catlin, the Blues are keen to add to their four summer signings ahead of the October 16 deadline.
It is understood a centre-half is now the priority, having finally captured somebody to fill the number 10 role in Michael Jacobs.
The Blues currently have Sean Raggett, Jack Whatmough and Paul Downing as their centre-back options, but are eager to bolster their numbers.
Catling told The News: ‘We’ll just have to see how things pan out over the next few weeks, but you're always in the market for players.
‘You can get an injury at any moment. Or you can get a player who, for one reason or another, doesn’t feel happy at the club, maybe not playing, and wants a move.
‘You always have to be on the lookout for new players – and we’re already looking out for players in January.
‘These days you can’t have such big depth in your squad as you would normally because you’re limited this season to just 22 players that can actually be registered.’
Pompey have so far signed Sean Raggett permanently and added Callum Johnson and Cameron Pring to their squad.
Monday’s arrival of Jacobs following his release from Wigan represents their fourth recruit of the summer.
It was also a signing described as a ‘strategic decision’ by Pompey’s chief executive.
Catlin added: ‘It’s about whether you go with 22 players, all with an equal salary cap, or do you make some strategic decisions in terms of one player may take up two slots, while another might take up one.
‘With the strength in depth of our current squad, there was a feeling that we should start working on some key positions and take that gamble.
‘Michael is a great addition and I look forward to him pulling on the blue shirt.’
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