The former Manchester United man had started all 13 of the Blues' League One matches prior to being dropped to the bench for last weekend's draw at Accrington.
After arriving on a free transfer in the summer from Luton, Tunnicliffe was Pompey's standout player in the early stages of the campaign.
He played a part in all of the Blues’ first four league goals, with three assists and a goal against Shrewsbury.
However, the Mancunian's performance level has dipped somewhat of late, which resulted in Joe Morrell and Shaun Williams being favoured in the deep-lying midfield roles as Cowley reverted to a 4-2-3-1 system at Accy.
Admittedly, after his superb early season form, Tunnicliffe was briefly utilised in the number 10 role, which could have played a part in disrupting his initial rhythm.
But Cowley has no doubt the former Hatter will capture those high levels of performance again as the campaign progresses.
And the Blues head coach highlighted what Tunnicliffe must now do in order to force his way back into his starting line up and produce his best on a regular basis moving forward.
Cowley said: ‘I think for Tunni, there’s no doubting his ability, he’s got very deceptive athleticism and he’s very powerful.
‘It’s just trying to find the consistency - that comes from making sure he approaches training and games in the right way.
‘I think for all professional footballers they have to be able to approach training with a focus, energy and intensity.
‘If you show that during the week, your training affects your habits and your habits then come out in a game.
‘You have to train intense everyday.’
Despite utilising Tunnicliffe in an advanced role at times this term, Cowley said the 28-year-old’s best position is in central midfield.
But the Blues boss stressed the player must be able to adapt to playing a variety of roles rather than a fixed position in the middle of the park.
Cowley added: ‘The nature of the three midfield roles, they can rotate in possession and out of possession, it’s a pretty fluid game in there so you have to be able to adapt.
‘He’s a central midfielder, somewhere in that central midfield (is his best position).
‘He’s got to be able to adapt across the positions and I think he’s someone who can.’