After last weekend's 10-9 defeat at the hands of Connacht, the defending champions will be desperate to bounce back at the RDS on Friday night and pull themselves up the GUINNESS PRO12 league table.
And Fanning, 28, who was part of the Leinster side that lifted the PRO12 trophy last season, believes victory over Cardiff will set them up perfectly for their showdown with Munster at the Aviva Stadium the following weekend.
"Obviously there's no hiding it, the Aviva Stadium game in two weeks' time is huge," said Fanning.
"It's obviously a massive derby. I know there's already a huge amount of tickets sold but for us we can only focus on this week. We can't win the Munster match this week.
"We've got to focus on beating Cardiff because if we start looking at Munster, Cardiff will come and do a job on us.
"All our focus as a squad must be on Cardiff. We want to be on good form going into the match against Munster - that's going to be huge.
"In the first twenty minutes against Connacht I thought we were very good. We felt comfortable out there but at the end of the day the only stat is the result and they won. There's lots of things we have to fix."
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