He was replaced after 20 minutes of Munster's Heineken Cup win over Benetton Treviso last month, complaining of dizziness, and his availability for the Test series will not be known until later in the week when the medical investigations are expected to be completed.
Declan Kidney explained: "There are ongoing investigations and, not being a medic, I wouldn't know (exactly what is wrong).
"I know that Marcus is not able to train and it is just prudent. He is just getting looked after and as soon as we know more we will let you know.
"In this job you can't afford to rule guys out until they are definitely out. He would need to be active next week to be involved with it.
"The investigations are taking place and the latest that we have is that they will have something concrete on Friday. I would only be adding to the rumours if I was to come out and say something now."
Meanwhile, Kidney revealed that the Irish medical staff have had a number of injuries and ailments to deal with over the first two days of the camp.
"We have a number of players who are at different stages - Tomas (O'Leary), Paddy Wallace and Denis Leamy have colds.
"We got through training okay this morning but we just had to work our way through it. With the lads playing last weekend, and that's what we would have wanted as you don't want to go too long without a game, but you don't want to cut it too short either.
"Rob (Kearney) was out, Gordon D'Arcy played a little bit of a part this morning as did Brian (O'Driscoll). The non-contact stuff basically for the two of those.
"Tony Buckley is rehabbing away (on a calf injury) and that's probably one of the more long-term ones. It will probably be a week or so before he gets back into it.
"Jerry (Flannery) didn't tog out today, Leo Cullen is rehabbing and Paddy Wallace will hopefully get back into things tomorrow."
Ireland's current 39-man squad will be whittled down to 30 players on Friday, with the team and replacements to face Australia being named next Wednesday afternoon.