The Blues, impressive 36-20 conquerors of Wasps last weekend, can eliminate Saracens from semi-final contention by winning at Vicarage Road.
And Gaffney accepts the Guinness Premiership club must produce a vastly-improved display following their surrender against Pool B rivals London Irish seven days ago.
"The display against London Irish was not acceptable, and that was disappointing bearing in mind the marked improvements we had made in our recent matches against Newcastle and Sale Sharks," said Gaffney.
"The fact, we made nine clean line breaks, which is a large number by anyone's standards, was pleasing.
"It is just that we didn't have the accuracy to finish it off, and that is the major disappointment."
Gaffney makes several changes from the 36-24 Madejski Stadium defeat, with former South Africa scrum-half Neil de Kock making his debut, while locks Simon Raiwalui and Iain Fullarton return, although Dan Scarbrough, Kevin Yates and Hugh Vyvyan are all rested.
Andy Farrell is again on bench duty, covering the back-row positions, and after a week of posturing between England and his club about exactly where he should play in the union code, he will be grateful for any game-time Gaffney allows him.
Blues head coach David Young retains the side that accounted for Wasps, apart from centre Marc Stcherbina - who returns to midfield duties instead of Tom Riley - and Gary Powell, who will feature in the front row.
England World Cup winners Lawrence Dallaglio and Phil Vickery both make their first appearances of the season against Adams Park visitors London Irish on Sunday.
Dallaglio starts in his familiar number eight role after having a plate removed from the ankle he injured during last year's Lions tour of New Zealand.
And Vickery, who signed for Wasps from Gloucester five months ago, features in the front row alongside Tim Payne and Jonny Barrett. He has not played since undergoing a third back operation.
The Wasps line-up also includes teenagers Danny Cipriani and Dominic Waldouck, with Cipriani hoping to make an impact after being called up for England training at Loughborough University earlier this week.
Wasps must win to keep alive any chance of retaining Anglo-Welsh silverware, but Irish will parade a strong line-up that includes seven internationals under the captaincy of fly-half Barry Everitt.
"Wasps will have been stung by last weekend's defeat in Cardiff, and they will be out to get their defence of the cup back on track," said Irish rugby director Brian Smith.
"In scoring five tries against Saracens, we lifted our performance another notch, but we will have to lift it even more if we are to come away with a result against the cup holders on their home ground."