And despite an ultimately convincing win, Nacewa wants his side to continue their winning form while improving their composure when points are on the line if they are to make it out of a highly competitive group.
"We work hard in training, and I think we are getting better each week, but it's about executing under pressure and finishing those opportunities," Nacewa explained.
"That's what we have to get better at if we want to progress through this tournament.
"It's pretty frustrating when we leave so much out there.
"When we review a game and see what we left out there, it's a lot for us to work on, but it's a positive sign that we still got the result."
Cullen has named O'Brien, Sexton and Healy in the same starting XV for the first time since before last year's Rugby World Cup.
And the former captain has acknowledged that his side are progressing, but knows they must continue that upward curve if they are to down Montpellier - who currently sit second in the Top 14.
"You're looking at all the opportunities you didn't convert and asking why we didn't convert them," he added.
"That's been a big focus for us this week. All the time we're just trying to get better, so whether that delivers the actual result. You can't exactly guarantee what our opponents are going to deliver.
"We just need to focus on how we get better as a group. I think we're making some good progress but the exciting thing for us as coaches is that we see a huge amount more potential in this group of players."
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Guinness PRO12 Leinster News
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