The Wales youth international was believed to be on Danny Cowley’s radar in January, before the Fratton chief moved for Coventry’s Tyler Walker.
But with Walker now back at the Ricoh Arena, and fellow loanee George Hirst, it’s rumoured the Blues could move for Cullen’s services once again.
At present, the south coast side have no senior players in the forward department, with Aiden O’Brien yet to agree fresh terms with the club.
This leaves just third-year scholar Dan Gifford as the only striker option at present.
As a result, a swathe of names have been touted with a move to PO4 this summer – including Cullen’s Swans colleague Morgan Whittaker.