The Blues currently possess a first-team squad consisting of 27 senior players – a tally the manager is determined to drive down.
In terms of loans, this month Jayden Stockley has returned to Bournemouth, although Matt Clarke and Marc McNulty are to stay for this campaign’s duration.
Meanwhile, Pompey are keen to also extend Caolan Lavery’s stay.
Instead, Cook is seeking to reduce numbers of those contracted at the club, primarily members whose deals end in the summer.
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The contracts of out-of-favour quartet Michael Poke, James Dunne, Nigel Atangana and Rommy Boco all expire at the season’s end.
And Cook has revealed he has already informed a number of unnamed players they will not be receiving new deals.
He said: ‘We want our numbers to come down, we feel there are too many players to look after.
‘What we are concentrating on at the minute is reducing the squad, obviously with one eye on the transfer window.
‘The problem is we have too many players not playing. The reality is they must leave. If I am not picking them then they must leave. If they don’t want to leave then don’t leave, that’s not my issue.
‘Come the transfer window when the likelihood is you are not getting a contract here, then players seem to want to leave a bit quicker because lads like the security of a longer deal.
‘There have been one or two cases when I have told players they won’t be getting a new contract. There are also one or two cases where you are trying to look after the player to highlight what is coming.
‘If a player has any sense he wouldn’t want his contract running out because nowadays there are a lot of players out of work in the summer.
‘If you look at what happened last year, it’s not a criticism, but there were 13 out of contact. In my opinion that is not correct.
‘When we have a player at the club we think is good I believe he shouldn’t get into the last year of his contract, we should be continually looking to renew.
‘Lads can let their contracts run out if they want, but I am not a great believer in that.
‘I think, as a club, if you have assets look after them and always try to get better ones in.’
In addition, Matt Tubbs is expected to leave Fratton Park this month, although is contracted until the summer of 2017.
Cook added: ‘We are trying to move one or two players out, we are trying to strengthen our squad as we go along. It might happen, it might not, we will wait and see.
‘Contracts must be respected by everyone, we don’t disrespect any players or treat lads differently. We want lads to just work hard every day they come in, to do their best until they leave.
‘The important message from me is we are not the finished product, just keep believing in what we are doing and let us do our jobs working out a way to make that better.’