Faletau has endured a stop-start campaign, withdrawing from the autumn squad before suffering a calf problem in early January that ruled him out of the opening three Six Nations matches.
There had been hope he might be close to featuring this week, but that timeline has slipped again.
"He's not training with the team yet," said Cardiff coach Corniel van Zyl.
"He's been up and running a few weeks now.
"(He's doing) the S&C and the medical stuff. So hopefully sooner rather than later."
Faletau’s continued absence compounds a back-row crisis at the Arms Park. Alex Mann and James Botham are away with Wales, while Ben Donnell and Lucas de la Rua are also sidelined.
"Lucas had that bad knock against Ulster in the back end of the game just after he came on, so that's a pity,” added van Zyl.
"I think we'll have to look after him for quite a while. We're uncertain on exactly how long.
"He's symptom-free so that's a positive.
"Ben Donnell is another one we had expected to be back at this time, but he had a setback. He's going to be out for a few weeks, definitely.
"Toby is obviously a little closer to returning to play and then obviously there's the other people away with Wales."
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That leaves Taine Basham, Dan Thomas and Alun Lawrence as the only recognised senior back-rows available, although academy players may be pressed into service.
"There's a few boys who can cover there," van Zyl added. "It's probably not the ideal position for them. But we'll have to make a plan."
Cardiff are boosted elsewhere by the return of Mason Grady, Ben Thomas and captain Liam Belcher from Wales duty, while wing Tom Bowen could feature with Josh Adams on Test duty and Iwan Stephens injured.
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Leinster arrive in formidable form, chasing a 12th straight win and strengthened by the returns of Robbie Henshaw and Rabah Slimani.
Meanwhile, van Zyl has acknowledged discussions with Benetton about their head coach vacancy, though he insists he hopes to remain in the Welsh capital.
"It's probably not true that I have had an interview with them but I have had conversations with people at the club," he said.
"It's not my decision to stay on in the future and ultimately you need to look after yourself and your family.
"I love Cardiff, I don't want to move anywhere else at this moment in time so we will see what happens. Hopefully I will have more clarity soon.”
