Tuilagi scored one try and set up two others in England's record 38-21 triumph over the world champions but he is struggling with an ankle injury. The Leicester centre was withdrawn at the last minute from the Tigers side that beat Toulouse in the Heineken Cup on Sunday and Lancaster is sweating on his fitness.
England have already lost flanker Tom Johnson for the whole Six Nations, while prop Alex Corbisiero and fly-half Freddie Burns will both miss the start of the tournament. "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 said.
"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."
If Tuilagi cannot prove his fitness, Lancaster's most likely move would be to start with either Jonathan Joseph or Billy Twelvetrees alongside Brad Barritt.
Lancaster would also have the option of bringing Toby Flood back in at fly-half and shift Owen Farrell into the centre, where he started his England career 12 months ago.
The decision Lancaster makes about his centres could influence who starts at full-back. Alex Goode gave England a second play-making option during the autumn, but the onus could shift to a strike runner like Ben Foden if Farrell or Twelvetrees got the nod in midfield.
Goode has one more chance to nail down the position. He has recovered from a shoulder injury and will play for Saracens in the LV= Cup match against Cardiff Blues on Sunday.
"It is quite an interesting selection debate," Lancaster said at Wednesday's Six Nations launch. "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."
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