While Farrell's place for next week's clash away to Ireland appears to be assured, Twelvetrees' spot is far from certain as England expect to have Tuilagi back from his ankle injury.
"It is a big step up for any player to make his debut, particularly at Twickenham," Lancaster said.
"We are delighted with the way [Twelvetrees] took it to get the try, but his confidence and composure has been good all week. It shows he is ready to make the step.
"Across the board, and with Manu coming back into consideration, there will be some selection decisions to make," Lancaster added ahead of the match against Ireland, who started their Six Nations campaign with a 30-22 win in Wales.
"Ireland are an outstanding side. We need to make sure we are ready mentally physically, and mentally technically," said the England boss.
England's other tries came via wing Chris Ashton, lock Geoff Parling and replacement back Danny Care. Meanwhile Scotland's New Zealand-born wing Sean Maitland crossed on his debut before Stuart Hogg grabbed a consolation try 10 minutes from time, with scrum-half Greig Laidlaw kicking two penalties and a conversion.
But it was the composed man-of-the-match display by Farrell, still only 21-years-old, that caught the eye of England attack coach Mike Catt.
"The mental toughness that he possesses was very noticeable," said the former England utility back.
"He was cool and calm under pressure and I thought he attacked the line very well. We look at putting the ball to where the space is and Owen did that."
That Farrell had so much good ball to work with was down to the dominance of England's pack and Scotland interim coach Scott Johnson said his side's failure to win the breakdown battle had been the key factor in their defeat.
"We can dream away about how we'd like to play the game of rugby but the reality is in the modern game, if you don't get the contact area right, you can dream all you like, it's fantasy, fairytales won't come true," said the Australian after his first game in charge of Scotland.
Scotland now face Italy at Murrayfield next week where they will look to avenge the 13-6 loss in Rome that gave them last season's wooden spoon.
"We need to get up," said Scotland captain Kelly Brown. "It's as simple as that. You can feel sorry for yourself but it's not going to achieve anything. It's up to us to learn our lessons, work hard and make sure we come out firing."
This article appeared in the South China Morning Post print edition as: Twelvetrees' performance prompts selection dilemma
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