And the Aberdeen boss revealed the winger is still mulling over remaining with the Scottish Premiership side.
Kenny Jackett is keen on bringing Mackey-Steven to Fratton Park on a free transfer, with a move expected to be completed this week.
McInnes has accepted the 28-year-old could leave the Pittodrie Stadium when his contract expires at the end of the month.
However, the Dons manager admitted Pompey are not the only club who are chasing Mackay-Steven’s signature.
And McInnes hasn’t given up on the Scotland international penning fresh terms with Aberdeen as he’s yet to ‘close the door’ on them.
‘Gary said he's not having a medical, but he's certainly having a look at one or two things,’ McInnes told the BBC.
‘I believe Gary is talking to one or two clubs and he's got options.
‘If he says he really wants to be here when he weighs up his options then we can look at that, but I'm not stopping preparing.
‘We've not closed the door on him and he's not closed the door on us.
‘He's got one or two things he's going to consider, but as recently as yesterday he was still telling me he is definitely considering staying – but I need him to want to stay.’