That is the expectation of Trevor Birch, who has revealed Reading have yet to table a bid.
The experienced Blues midfielder moved on loan to Brian McDermott’s side in March.
Mullins went on to feature seven times to help the Royals take the Championship title.
Reading have the option to make the deal permanent.
As of yet, though, there has been no indication they will be pursuing this.
And Birch believes Mullins is heading back to Fratton.
He said: ‘Reading have not indicated they want to take him.
‘There has been no bid or anything so that is very much in the mix.
‘I would say, for all intents and purposes, Mullins is coming back.
‘Of course that may change, but at the moment I don’t get the impression it will.’
Mullins was one of four first-team players shipped out on loan from Pompey during the emergency loan window to ease the club’s wage bill.
Stephen Henderson will make his move to West Ham permanent once their play-off campaign is over.
Despite having yet to feature for the Hammers, he is seen as having a long-term future with Sam Allardyce’s side.
The situation with Liam Lawrence is expected to become clearer following Cardiff’s elimination from the play-offs last night.
The Republic of Ireland international has been a first choice since joining the Bluebirds in March.
It remains to be seen whether they are to turn the switch into a permanent one.
Finally, Erik Huseklepp remains on loan at Birmingham.
Chris Hughton’s men face Blackpool tonight in the second leg of the Championship play-off semi-final.
The Norwegian international has struggled to nail down a regular spot in the side, making just four starts.
It is unlikely Birmingham will look to sign the forward, although SK Brann continue to show interest.