The Blues boss confessed he has only 18 fit senior players for the League One game, with Ryan Tunnicliffe joining Clark Robertson, Liam Vincent, Paul Downing, Jayden Reid and Ellis Harrison in the treatment room.
Tunnicliffe had to be substituted during the 1-0 win against Bolton on Saturday because of a hamstring injury, with the club’s medical team awaiting the results of a scan.
That’s set to rule him out for the game against the Robins.
But Cowley expects Joe Morrell to be fit and available after a groin injury prevented him from starting against Ian Evatt’s Trotters.
And with Gassan Ahadme likely to return to the match-day squad after a period in the stands – along with the availability of some of the academy’s bright young prospects – the Pompey head coach refused to get downbeat as the injuries pile up
Providing the latest on the injury front, Cowley said: ‘Joe trained today so we're hopeful he's going to be available from the start.
‘Like I said on Saturday, it's just been a niggling groin which, probably with two games in close succession, might have been a bit too much.
‘But we're hopeful that we will be available to start (against Cheltenham).
‘Ryan is having a scan today and we'll get the results back this evening.
‘We're hopeful that it's more neural/nervy rather than soft tissue and muscular.
‘We've only got 18 for bodies for tomorrow.
‘Young Gassan will come back into the squad, which is good.
‘The injuries are mounting up, but what that does do is give an opportunity to someone else and it also gives an opportunity to some of our young players – you know, young Izzy Kaba, Harvey Hughes, Harry Jewitt-White and Toby Steward.
‘They've all been training with the first team regularly and it will open the door for one of those - which us great for the academy and great for the future of this football club.’