The American will be on the bench for Saturday's curtain-raiser against Shrewsbury at Fratton Park.
Alex Bass has been ruled out with an ankle injury, meaning Turnbull will play back-up to Craig MacGillivray.
New salary cap measures implemented this season restrict clubs to having just 22 players above the age of 21 registered.
The Blues have been planning to send Turnbull out on loan to free up a berth in their squad and get him senior experience.
That’s been shelved for the meantime as the former Notre Dame keeper is required for first-team duty.
But Kenny Jackett revealed Pompey's 22 players only have to be confirmed after the transfer window closes on October 16.
As a result, there will still be two places still available in the Blues’ squad even when Bass returns to fitness.
Jackett said: ‘We will register two of the three goalkeepers. We're working on that basis and will have a squad of 20, which isn't an excessive number.
‘You don't need to enter your squad of 22 until the transfer window closes.
‘I don't think you've got a lot of flexibility after that but up until that point, my understanding is it's when the window closes when you need to make decisions.’
Bass sustained his injury in last month's penalty shootout win at Stevenage in the Carabao Cup.
The academy graduate suffered bad bruising on his right ankle, which is still to clear up.
Pompey are hoping he'll be back training at the start of next week.
Jackett added: ‘Alex's injury has been lingering. I only thought it'd be for a few days but it's pretty sore and we do hope by the start of next week he will be okay.
‘It's bone bruising after a collision in the box. Stevenage have some big lads and it was quite innocuous but has been pretty sore afterwards.
‘Hopefully it settles down pretty quickly.'
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