The Samoan-born centre was a late withdrawal from Leicester's Heineken Cup win over Toulouse on Sunday and now faces a race against time to feature at Twickenham.
Lancaster is keen to finalise his preparations prior to the start of the final week of training ahead of the Calcutta Cup clash but he remains optimistic that Tuilagi will be available for selection.
"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," said Lancaster, at the Six Nations launch in London.
"Richard (Cockerill, the 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."
Regardless of whether the 21-year-old midfielder makes it back in time for Twickenham, Lancaster insists he will have plenty of difficult selection issues for Round 1.
Despite recording a famous win over the All Blacks in their last outing, the England boss says picking his matchday 23 isn't simply a case of sticking with the side that created history on December 1.
"It's quite an interesting selection debate," added Lancaster.
"I don't think selection going into the Scotland game is as simplistic as people perhaps think on the back of the All Blacks game.
"We've got a lot of decisions to make: Ben Foden is back, who obviously didn't play in the autumn. Dylan Hartley is back (from suspension). Dave Strettle is in. There's pressure and competition in every position, which is great."
A host of injuries has already made naming the same squad impossible, with prop Alex Corbisiero and replacement fly-half Freddie Burns both definitely set to miss out.
Corbisiero is struggling with a long-term injury that has seen him play next to no rugby for London Irish this term, while Burns faces two-three weeks on the sidelines with a knee injury of his own.
Newboy Calum Clark and Exeter flanker Tom Johnson - neither of whom played against the world champions - won't be in the mix either, as both were ruled out for the entire tournament after picking up respective shoulder and knee injuries in Heineken Cup action at the weekend.
Lions blindside Tom Croft is also uncertain of his involvement after Cockerill admitted his club side were still trying to manage his comeback from a neck problem, while Saracens full back Alex Goode will have to prove his fitness after suffering a shoulder injury.
"It's already impossible to select the same XV because Corbisiero will miss the first two Six Nations games after a procedure to clean out his troublesome knee.
"Burns misses the Scotland game and maybe Ireland, also, because of knee-ligament damage.
"Now Tom Johnson is probably facing a three-month recovery time. So we've lost two flankers in a matter of days.
"Alex has made very good progress and will play for Saracens against Cardiff on Sunday. We want him to get game time before making a decision on his availability for our opening game."