Pompey will field a young side at Oxford in Tuesday night's Leasing.com Trophy fixture.
The competition holders have qualified for the knockout stages with one game to spare, having already beaten Crawley and Norwich U21s.
And they will look to secure top spot in Southern Group B – ensuring home advantage in the second round – at the Kassam Stadium.
Ronan Curtis and John Marquis are both unable to feature, but Kenny Jackett believes the latter should be fit for the weekend.
The Blues boss said: "We’ve qualified already and it will be a very youthful side at Oxford – it’s important our supporters understand that.
"Ronan Curtis has got a tight hamstring that kept him out against Doncaster and saw him withdraw from the Republic of Ireland squad.
"We’ve qualified already and it will be a very youthful side at Oxford – it’s important our supporters understand that."
"He’ll have a scan this afternoon and we’ll have to see the results of that, but hopefully he won’t be out for long.
"John Marquis has a bruised toe from Saturday and he should be okay for Gillingham, but won’t be involved against Oxford.
"We took 19 fit players to Doncaster and unfortunately someone had to miss out, which was Andy Cannon in this instance."