Leinster are waiting on more results on RG Snyman's injury but expect to be without the influential lock for a "few weeks", while James Lowe is still four to six weeks from fitness.
Double World Cup-winner Snyman suffered a knee injury while being tackled by Glasgow's Alex Samuel in last weekend's URC defeat.
"He's injured his knee so we'll get a full report out but he's going to be out for a few weeks unfortunately," Cullen told RTÉ Sport ahead of tomorrow's visit of Scarlets to Aviva Stadium (7.45pm).
The 31-year-old lock suffered two serious knee injuries while at Munster between 2020 and 2024.
Cullen also told reporters in a separate briefing that he would be surprised if Snyman returned this season.
The head coach has made 12 changes to his side for the game against the Welsh side, with Joe McCarthy coming in for Snyman.
Winger Lowe injured his groin while playing for Ireland against England in February and the 33-year-old is also expected to miss a significant chunk of the run-in.
"James, he's another four to six weeks, I'd say, so somewhere in around there," said Cullen.
Leinster have lost their last two league games, to Warriors and Cardiff, and have not lost three in a row since October 2008.
There's a return for prop Andrew Porter (below), who has not played since suffering a calf injury in January, while Ireland internationals Dan Sheehan, Tadhg Furlong, Josh van der Flier, Caelan Doris, Jamison Gibson-Park and Jamie Osborne are also included in the starting XV.
"We're at the end of the URC regular season. I think everyone knows we need to battle it out from every single point that we can," added the head coach ahead, whose side sit fourth in the table after 13 games.
"We come off the last week, we get nothing from the game and everyone has that very hollow feeling.
"A lot of the guys weren't over there. They're not necessarily sharing that same feeling but they understand it from a bigger picture point of view.
"Andrew hasn't played in a while so it's great to see him just back up and running.
"There's a tonne of experience in the group. So yeah, those guys, I don't need to tell them specifically how to suck eggs, do I?
"It's just making sure they are able to get control of the group quite quickly.
"They're all back aligning together as to how we want to play the game. That's all the different facets of the game.
"But that's the challenge.
"It's a short period that we have in terms of preparation for this game. Scarlets have been building a bit of cohesion for the last few weeks because this is their third game in a row pretty much together.
"A couple of different changes but they're a team that can really punish you. You can see they've scored some good tries at the weekend [a 36-17 win over Zebre] in their last game.
"They have that bit of cohesion that probably we don't.
"You look at our team on paper, yeah, it looks strong on paper. But listen, it looks strong on paper, but the actually reality of the game is in the performance."
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