There are still six places up for grabs in the starting XI as Pompey approach their final pre-season friendly.
The Blues face Rayo Vallecano on Sunday ahead of the August 3 League Two kick-off.
Guy Whittingham has revealed five players have already booked their places in his line-up against Oxford.
Although he remained tight-lipped on names, that will include Johnny Ertl, Joe Devera and David Connolly.
The Pompey boss insists other spots are still to be filled as he seeks to finalise his preferred team.
And he is adamant there is time for contenders to make their mark.
Whittingham said: ‘I am very satisfied with how the players have done pre-season.
‘The test will come when you pick that first XI for the first game of the season, to see how everybody stands up to being either selected or not selected at the time.
‘I wouldn’t say there are a lot of places decided, although you look at it and have some general ideas.
‘Nothing has been firmed up and the staff are always talking about it.
‘I would say probably five are certain.
‘But I think that is down to the lads because they are making it difficult to make it an obvious choice.
‘We are looking at every position and every player, there is competition for places and it is going to be a hard decision.’