The Leicester Tigers centre, who inspired his country to a 38-21 win over New Zealand last month, is struggling with an ankle injury.
Tuilagi was withdrawn from the Tigers side that beat Toulouse in the Heineken Cup last weekend after picking up the injury late in the game.
"I need a decision to be made early, by the weekend preferably, because if Manu is not available then there will be decisions to be made," Lancaster told reporters.
"Richard [Cockerill, Leicester director of rugby] said he was 70-30 in the lead-up to the game and they pushed him.
"Manu is pretty optimistic that he's going to be alright but I don't want to say now that he's going to be fit or vice versa. We'll just have to wait and see how he gets on. It's too early to tell."
Lancaster is facing mounting injury concerns ahead of the competition, with flanker Tom Johnson due to miss the whole tournament and prop Alex Corbisiero and fly-half Freddie Burns carrying short-term injuries.