The 24-year-old had to be carried off on a stretcher after injuring his ankle in the closing stages of the Dubliners' 29-28 quarter-final triumph over ASM Clermont Auvergne and hasn't played since.
Also in the same boat is Irish international outside half Johnny Sexton, who fractured his jaw after providing a match winning 19 points. A decision on his availability will be taken on Thursday.
"We're be a lot more positive about Rob after training today, but only way that Jonny Sexton will be playing is if we feel that he can do his job to the fullest," said Leinster coach Michael Cheika.
"If we have to play him, and hide him out the back or something like that, then he won't be playing. If he's 100% confident that he can play, then he will.
"It's not like a hamstring tear or a calf strain - it's pretty cut and dried. It's going to come down to a bit of risk assessment; knowing what the ramifications could be and everyone being on board with that and then doing it if we decide to go down that route.
"I'd say we're pretty comfortable with both situations. If he plays, fantastic. If he doesn't, then we'll look at the option and it's pretty much certain that we'll have him available for the rest of the year.
"We'll make that call on Thursday and it will be a qualified decision based on the information that we've got. We're a bit in the dark, but I suppose the opposition are in the dark as well."
Cheika is more confident about having 2005 Lion Shane Horgan back in his starting line-up following a slight niggle. He is also hopeful that Girvan Dempsey and Fergus McFadden will all have shrugged off knocks.
With a potential double in their sights - Leinster are also top of the Magners League and set for a home semi-final in the Play-Offs - Cheika knows he needs a full strength squad to meet the challenges ahead.
"We know that we're going to face the sternest test of our whole time together in Toulouse. It's the biggest battle that we have ever come across and we'll have to dig in and fight hard because we're a couple of men down," admitted Cheika.
"To do that will take a real team effort. We're quite anxious for the game to get going and we've had a good preparation in the first few days of the week.
"Toulouse have been pretty clear that Europe is their priority and their coach has been talking all week about paying credit to the players for winning last weekend's game in the Top 14 against Castres.
"They fancy their chances against us deep down and they're probably pretty happy that they have drawn us. They see that as the quickest road to get to Paris.
"Their forwards coach has added a lot to their team and their programme is very profiled. In their back-row, for example, they know that have a strict ball carrier, a strict line-out operator and there's a strict tight player.
"They know their profiles and, once they put their combinations together, it will be interesting to see the combinations they select. They will then give us an indication of how they are thinking about playing the game."