But having battled to four wins out of six so far and with many of their Rugby World Cup stars back fit and firing, Cullen's troops are only likely to get stronger as the season grows on.
Tonight will be the acid test, as Leinster welcome an unbeaten Scarlets to the RDS, and Cullen admits staying within the top four is harder than ever.
"It's a tough challenge this year because the big games have come around really, really quickly," he said.
"Because of the challenge presented in the league, that's again something that we need to be really mindful of because we can't be in a situation where we were last year trying to chase results later in the season and we were struggling to get into the top four.
"We have just nipped into the top four, whether we stay there, we're playing a team that's just won six from six so it's a huge challenge for us on Friday."
The GUINNESS PRO12 breaks up temporarily for Europe next week and Cullen says he needs to perfect the balance between both competitions after last year's misstep.
He added: "We're always looking for different ways in how we can manage our season but we struggled at times last year.
"We went hard at Europe and it had a knock-on effect where we had a disappointing league campaign.
"The challenge for us is to balance the two tournaments. We need to be careful because of the qualification process, we need to make sure we are in a good position in the league and learn lessons from last season."
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