Billy Vunipola scored a six-minute hat-trick, Ben Foden touched down twice and there were tries from Tom Wood and Paul Doran-Jones as England rallied from an error-strewn start.
England face Argentina in the first of two Tests in Salta next Saturday and Lancaster said: "In terms of preparation it has been ideal. They were difficult opposition and we struggled in the first quarter and the last quarter to get the continuity we wanted."
He added: "We were pleased to be 31-7 ahead at half-time after a slow start. In the second half our set-piece ball dried up a bit.
"The most important thing for me (from this game) is that it is now crystal clear exactly how difficult it will be next Saturday."
Vuniopola's hat-trick on his first senior England start capped a memorable weekend for the family, coming the day after his elder brother, Mako, had made his debut for the British and Irish Lions.
But with Ben Morgan set to start at number eight and England acutely aware of the physical battle that lies ahead, particularly at the breakdown, Vunipola is not even guaranteed a place in the squad to face Argentina.
"He had good game in terms of try-scoring," Lancaster said.
"I need to look at his contribution and those of all the others in back row because there are lots of decision to be made.
"The breakdown was fiercely contested and at times we didn't deliver as good ball as we had wanted. I have some decisions to make as to how we go about the game."
- Sports & Recreation
- Billy Vunipola
- Ben Foden