Grant McCann is wary of Pompey's goal power.
And the Doncaster boss believes Brett Pitman's recent stint on the Blues bench shows the strength of his side's opponents tomorrow.
Pompey are out to equal their best start to a league season at the Keepmoat Stadium but Donny's opening has also been strong, winning three and drawing one of their first five games.
McCann knows his side are in for decent test.
He said: 'They're scoring goals aren't they?
'They're that strong that they can leave Brett Pitman on the bench. So we know it's a tough game.
'They're quick on the break leading to a low block and they make sure you have to pass the ball quick and in between them as much as you can.
'We're going to have to be patient at times.
'Kenny's sides are always hard to beat.
'My Peterborough team beat them 2-1 last year at London Road and we knew beforehand how tough it would be.
'It's a big test for us but, as always, we're going into it looking to get the win.'