The Blues boss sent Milan Lalkovic out on loan to Scottish Premier League outfit on Friday.
Lalkovic found playing time increasingly limited under Cook and hasn’t started a league game since September.
Pompey have bolstered their squad with the signings of non-league pair Nicke Kbamba and Jamal Lowe.
And Cook feels those who aren’t getting playing time should seek it.
He said: ‘Lads will always be unhappy not playing.
‘If you look at our squad, is the necessity to get people out? No.
‘Will I get people out? Probably.
‘That’s the way it is. Lads must want to play football.
‘If you’re a lad here and you’re not playing football I feel there must be something wrong with you.
‘It’s not the hardest part because when you have good players in the group you can’t keep them all happy.
‘It’s important they gel into that. It’s our job. We can pick players, do our jobs and win.
‘We’ve used an amount of players in general, so the quicker the group get into that the better.’
Cook has made it clear he’s been looking for a ‘handful’ of a striker to bolster Pompey’s bid to get out of the division.
But the Blues boss explained the challenge is getting the right body through the door this month.
He said: ‘When you have strikers they are all different. How do you get a handful? Define a handful?
‘You’ve just got to be careful you don’t go through what you need to win a game.
‘All clubs want to progress at a rate. What you have to do as a manager is have each type of personnel in the building at the correct time.
‘That’s huge. Look back to Ryan Taylor here. Was he a handful? Yes. But the criticism of Ryan was injuries and not enough goals.
‘You are trying to get the jigsaw pieces in place.
‘Sometimes they may not be quite available when you want them.’
Cook stressed he’s pleased with the make-up of his squad at present - and the Pompey dressing room is in a good place.
He said: ‘Winning dressing rooms are the best place in the world.
‘Now, if you look at us, the amount of winning people we’ve got in the team We’ve had good performances.
‘They’re all different types.
‘Within us, we have a good blend of young players coming through and senior players.
‘The team spirit seems to be as good as its been for a while – long may it continue.’
– JORDAN CROSS