Pompey boss John Mousinho says the problems Port Vale caused his team at Fratton Park in October act as a warning this weekend's trip to Vale Park will be tricky.
The Blues won 2-0 with two goals early in the second half from Colby Biship after a tight first half.
Vale are 18th in the table but have won five out of 12 at home and Mousinho told the club media: 'If you remember the game at Fratton Park earlier in the season, they gave us a really tough 45 minutes.
'We had to adjust a couple of things at the break, although I thought we were the better side after that. But Port Vale have got a decent squad and they've been able to bring in some promising young players.
'They're well drilled and it's always a tough place to go, so we're expecting a decent contest on Saturday. It's tough to win games at this level – as we've seen from some of the results over the past few days.'
Mousinho is giving nothing away about whether he will again go with the new 3-4-2-1 formation he fielded for last week's win at Fleetwood.
He said: 'It's one of those things that players should be able to adjust to pretty quickly. Whether from being at other clubs or in academies, they should have experience of a number of different formations.
'I also think you should have that flexibility anyway – a centre-back that can play in a four should also be comfortable in a three. There aren't too many things that are dissimilar and we always work on different things in training.
'Having played with a back four for my first year here, I thought we did well at the weekend and I think it's something we can ask them to switch in and out of.'
Mousinho revealed Joe Morrell should be fit to face Vale but Joe Raffery was 'a bit more touch and go'. Meanwhile Ben Stevenson is out for three or four weeks with knee damage.