The January transfer window shuts at 11pm on Monday night – clashing with Pompey’s 7.45pm televised clash with the Addicks.
Danny Cowley has admitted he wants two more arrivals in the window’s final moments, not that he will be too involved on the day itself.
Instead Pompey’s head coach must focus on the encounter with Charlton as he chases a maiden League One victory.
Still, responsibility for negotiation with possible transfer targets will have already moved on elsewhere by the stage of deadline day.
That brings in Cullen, who may find himself called away from the directors’ box during the evening.
The Blues’ chief executive told The News: ‘If it works out that all our business is done the day before the Charlton game then that would be great.
‘However, the window shuts at 11pm on Monday and if the right opportunity is available, and we still need to get a deal across the line, we will work hard at it.
‘We definitely won’t be downing tools at 6pm on Monday to head into match-day mode!
‘Approaching the game, you are at the stage where you are basically doing the deals and the player has been identified, so the manager isn’t needed.
‘It’s not like suddenly I will sign a player without the manager's final say on those he wants to bring in.
‘Danny has identified the player and we are then trying to do the deal – successfully or unsuccessfully.
‘There will be myself and Tony Brown (chief operating officer) doing the deals and Anabel Roman (club secretary) the paperwork, so we can crack on until 11pm. If need be, we’ll be ready.
‘It’s just an extra dynamic to transfer deadline day.
‘Should there be more business to be done – and I’m not saying there will be – but, if there is, then hopefully we can tie most of it up by the time the match kicks off.’
Pompey have so far added Hayden Carter, Tyler Walker, Ollie Webber and Denver Hume to their squad this window.
Yet with Lee Brown the latest departure, the squad size has clearly reduced significantly overall.
Cullen added: ‘A transfer deadline on the day of a game has happened a couple of times over recent years, but not very often, it’s quite a rare occasion.
‘At Norwich, I can remember signing Robbie Earnshaw on deadline day in January 2006 while in the bowels of the Madejski Stadium.
‘Norwich were losing 4-0 to Reading at the time and Earnshaw was to be Dean Ashton’s replacement, who we had sold to West Ham.
‘The last time it happened at MK Dons, we had done our business anyway, so nothing was happening.
‘Certainly I have never known a game on August deadline day, except maybe a Johnstone’s Paint Trophy.’
A message from the Editor, Mark Waldron