Cullen believes the trip to Marseille will represent the toughest challenge of Leinster's season to date, but victory could give them the boost required ahead of the run-in.
"It's the biggest test the squad are going to face this season. What Toulon have done over the last couple of years has been incredibly impressive, backing it up with two trophies and in a semi-final again," Cullen told Leinster's official website.
"Everyone understands the challenge. From our end we haven't performed to the levels expected and that's hugely disappointing. But this is knockout rugby, one game. We just need to make sure that we prepare as best we can.
"The lads have a pretty good understanding of what's expected. We'll go there with a plan, will give it our all and that's all we can expect of the lads."
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