The Blues boss is toying with reverting to the system which has proven so successful this season.

Following the Middlesbrough defeat less than two weeks ago, he admitted he would head to Wembley operating with a 4-4-2 blueprint.

Now he appears to have had a change of heart.

And that will open to door for either Papa Bouba Diop or Pedro Mendes (below) to operate as the fifth midfielder.

Pompey test drove the 4-5-1 formation in training yesterday as the clock ticks down before Saturday's date with Cardiff.

And Redknapp conceded using Kanu as the lone striker is now utmost in his mind.

He said: 'It's a difficult week. I've got to pick a team and I've got one or two I might have to disappoint who have played in other rounds.

'I haven't said anything but they will know because we've been working with the shape of the team.

'They probably knew yesterday morning in training in all honesty.

'I've already done some work with the team so they'll have an idea.

'Our best form this year – and what has put us where we've been in the league and probably in the Cup as well – is playing 4-5-1.

'We played 4-4-2 on Sunday and didn't score. We played 4-4-2 against Middlesbrough the week before and didn't score.

'You can score goals playing 4-5-1 as easily as you can playing 4-4-2 really. When we got seven goals against Reading, which was our best goalscoring feat, we played 4-5-1.

'What happened against Fulham hasn't changed my mind.

'We've gone between the two systems a bit this year but with 4-4-2 our form hasn't been quite as good to be fair.

'I know what I want to do but, to be fair, I'm not going to say.'

Mendes – reported to be interesting Aston Villa boss Martin O'Neill – has emerged as a surprise candidate having featured against Fulham on Sunday.

He is now ahead of Sean Davis, Richard Hughes and Arnold Mvuemba for a midfield spot, while Diop has been a regular during the campaign.

But neither will play if Redknapp opts for a 4-4-2 with Niko Kranjcar, Lassana Diarra, Sulley Muntari and John Utaka definite starters.

He added: 'Pedro is in there with a chance of playing, but it's like everything, I don't want to show my hand too early to be honest.

'Dave (Jones) might decide they are going to play a way to nullify it so I'll keep it under wraps a little bit if I can.'