Cullen knows it's a difficult ask to keep all at Leinster happy
Of the 21 Leinster players who played in last month's impressive Guinness PRO14 win against a strong Glasgow side, only seven went on to feature in the Champions Cup over the last fortnight.
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That means the other 14 have spent the last weeks holding tackle bags and running as Northampton Saints in training.
A handful of the younger contingent got a run out in last week's A game against Munster, a major comedown after winning in Scotstoun.
It's the harsh reality of top-level professional rugby, particularly in a club that boasts such strength in depth as Leinster.
Leo Cullen used 55 players last season, which is a remarkable achievement given that Leinster only had 44 in their senior squad. That number has increased by two this year and, while such resources are the envy of most, Cullen has a major job on his hands to keep all of them happy.
A lot of that will come down to the ambition of the individual because some guys will truck along as bit-part squad players. It should be noted, however, there aren't many of them in this current set-up.
Injuries will always play a part in how deep a coach has to dig into his talent pool, yet the rate with which Leinster are producing young talents who harbour desires of playing regular Champions Cup rugby and beyond means that the balancing act isn't easily achieved.
Cullen has done a good job of it up to this point, but it is about to become much tougher.
Ultimately, the best players will remain with Leinster because they will be involved in the big days, but the ones coming behind are what helps make the province an efficient machine.
Take Joey Carbery for example. He wasn't happy playing second fiddle to Johnny Sexton and when he had his head turned by Munster, he jumped at it.
There haven't been too many examples as extreme, but if you look at the back-row options, it seems like an impossible task to satisfy all of them.
Will Connors was called up to the Ireland training squad this week despite not having played in the Champions Cup, which says a lot about the potential that Andy Farrell sees in him.
After performing so well in the PRO14, Connors was unable to force his way into the European reckoning. And when you consider Dan Leavy and Jack Conan still have to come into the Leinster back-row, the risk of a major log-jam is very real.
"They're desperate to play and kick on," Cullen said of his younger players.
"Will has had a couple of outstanding games but Josh (van der Flier) has come in and done very, very well. Will just has to bide his time now.
"Generally, their attitude is good. Whether they understand or not, it gets explained to them, 'So this is what the plan is, and this is why this player is getting selected'.
"I wouldn't say everyone is delighted with the news but we go through the process with them."
To Cullen's credit, he has created an environment where players feel that they can push on and improve.
Leinster are basically operating with two different teams at the moment though, which means that the door will again open for the likes of Connors over the coming weeks, starting at home to Ulster on Friday.
"If players turn around and say, 'I'm not happy because of this, this and this' then you have to respect those decisions," Cullen added.
"We can just focus on having good coaches and support staff."