Leinster have been drawn alongside French Champions Clermont Auvergne, English Premiership runners up Saracens and big-spending Racing Metro Paris in one of the standout groups in this season's tournament.
And while Leinster's European pedigree has made them many people's group favourites, Cullen admits they could have been handed a far less complicated route to the knockout stages.
"Our group is so tough in Europe that it's very hard to look beyond those first two games. If we look too far past that, we could be out of the tournament," said Cullen, who lifted the continent's greatest club prize at Murrayfield 16 months ago.
"I think we really got a pretty raw deal with the group we got. I think it's probably one of the toughest pools there's ever been.
"If you look at the level of Clermont, Racing with the way they're operating, and Saracens, they're three pretty tough teams to come up against.
"The first five games in the Magners League are about getting in a half decent position in the league and going into Europe in good stead. Hopefully, we'll then have a few more players back by then."
Whether Cullen himself is one of those players remains to be seen.
The hugely influential second row is currently recovering from a shoulder injury and is as yet unsure of a return date.
Cullen's rehab is progressing nicely and he hopes to be back at some point in October but he admits that he's taking things one step at a time.
"It's going pretty well. It's a bit of a tedious road - you have your good weeks and your bad. Some weeks you hit a bit of a wall and can't really push on and other weeks you're flying through," added Cullen.
"I'm reassessing every couple of weeks so it's hard to put an exact time on when I'll be back but I hope it will be sometime in October. That probably leaves me a bit tight in terms of getting back to Europe, but we'll see.
"When you start getting back watching games, it gets a bit frustrating going along as a spectator and it's not ideal.
"But it'll be a decent pre-season for me in the end. I'm just trying to push on now and see where that gets me to. I'm happy with the direction I'm going - I'd just like to get there a bit quicker."