Jermaine McGlashan remains the Robins' record £50,000 signing from Aldershot in January 2012, with Brian Wilson and Grant McCann costing the same almost 20 years ago.

The figure is dwarfed by the £64.3million City paid Benfica for defender Ruben Dias in September and Duff knows the game is a mismatch.

"It's the biggest free hit in history and then we move on," he said. "We will do everything we can to make it as difficult as possible.

"There will be a little bit of trepidation because they are not bad but there will be a nervous excitement. It's an opportunity to get away from the league, it has been relentless asking the players to go again.

"The players will get a bit of kudos and a bit of attention which will be nice for them. This is a bit of reward, a little bit of sugar and we'll crack on from Monday in the league as normal.

"We are not going to try to out-football them, we just have to give a really good account of ourselves and you never know what might happen, it's the FA Cup.

"We have things like a long throw and people who are good in the air – the key is can you get a throw in their half to start with?"

Duff could be without Chris Hussey (foot) while defender Lewis Freestone and goalkeeper Scott Flinders are doubts.

"You are in the FA Cup for moments like this, just give it your best shot. Will we win? Probably not – but you never know," said Duff.

"It doesn't matter who is playing for them, it's just numbers. You replace one for one, on Wednesday they took (Raheem) Sterling off and (Riyad) Mahrez came on.

"It's the same patterns and same movements, it's just a fresher pair of legs with the same quality.

"They are the form team in the country, if not the world, and just to add to that we haven't won in seven games."