And the Blues chief executive insists the club are under no pressure to cash in on their prized asset in January.
With the transfer window reopening on Saturday, Pompey are braced for Championship interest in the leading scorer.
The 25-year-old has nine goals from his last 16 appearances in all competitions, sizzling form which will undoubtedly attract admirers.
Harness is out of contract at the season’s end, yet, crucially, the Blues possess a club option to extend it by 12 months.
And Cullen insists owners Tornante have made it clear they want the former Burton man to remain, irrespective of potential interest.
He told The News: ‘If you have a player in the form of Marcus, then you may get one or two enquiries.
‘But we see Marcus very much as part of our football club going forward, he is hugely important to us.
‘This is a player that, earlier in the summer, we were told we might get some expressions of interest, but there was nothing in a position to interest us at the sort of value talked about.
‘We have the option on Marcus anyway. He’s somebody I know Danny very much wants to see through to the end of the season and continue the push.
‘We are looking upwards, we are not looking over our shoulder. We are looking upwards and where we can potentially get to, so you want a squad as strong as possible to see where you can add and where you can get to.
‘Obviously every player has a value and, at some point, you get to a stage with any player where the deal is too good for both parties.
‘It is very, very difficult at that point to stand in the player’s way. You get into a position which is right for the club and enables you to go and do something better.
‘You never get into a situation where you can close the door on anything, because sometimes it can be a little out of your control.
‘If you get silly offers or silly bids and a player gets a life-changing opportunity for him, then you have that discussion.
‘But we are under no pressure as a football club to do that.’
James Bolton was the last player Pompey sold, joining Plymouth in June on a two-year deal.
Although undisclosed, the fee is believed to be minimal – with Harness inevitably attracting a far greater price tag should clubs come calling.
Cullen added: ‘We are not like some clubs, where fire sales are needed.
‘We are very, very supported by the owners and they want to see success for the football club as much as any supporter does.
‘So we want to try as best as we possibly can and keep our most talented assets here, which then can give us a platform to build for the second half of the season.
‘This football club now is in a really good position to challenge for the play-offs – and who knows beyond that.’
A message from the Editor, Mark Waldron