The Blues were drawn away to their League One rivals last night after earning their place in the second round with a 4-0 thumping of Maidenhead on Saturday.
It means it is familiar foes for Pompey and they can take huge confidence from the 3-1 success there in the league.
On that day Jamal Lowe, Brett Pitman and Matt Clarke all netted.
Assistant boss Gallen felt that performance was a really strong away showing and one the league leadersÂ must look to repeat in the Cup.
He said: '˜If we play as well as we did on that day we will be in with a good chance of getting through.
'˜It was a convincing win 3-1. It is a place we have played well at and got a really positive result.
'˜We will respect them, we know it will be a tough game but we will have a lot of belief.'
Unlike when they played Maidenhead there won't be as much study for boss Kenny Jackett and his coaching staff to do to swot up on the opposition in quick time ahead of the tie.
Gallen added: '˜It's a bit more straightforward. We know more about the opposition.
'˜For Maidenhead we had to do a lot of home work. Talk to people. Look at them and find out about their players.
'˜Rochdale will be more straightforward. We know about them and they know all about us as well.'
The FA Cup second round matches will be played on the weekend from Friday, November 30.