England boss Stuart Lancaster admitted his side were given a wake-up call in Sunday's 41-21 victory over a South American XV.
The win was set up through a hat-trick of tries from Billy Vunipola, a brace of five-pointers from Ben Foden and further touchdowns from captain Tom Wood and Paul Doran-Jones.
But things did not go their way for the entire match and the CONSUR XV - which consisted of players from Argentina, Uruguay, Brazil and Chile - dominated the first and last quarters of the match.
England also battled at the breakdown throughout and Lancaster knows his charges will have to improve considerably if they want to beat Argentina in Saturday's Test in Salta.
"In terms of preparation it has been ideal," he said.
"They were difficult opposition. We struggled in the first quarter and the last quarter to get the continuity we wanted.
"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.
"The most important thing for me (from this game) is that it is now crystal clear exactly how difficult it will be next Saturday.
"There are lots of selection decisions to be made."
Alex Corbisiero was sent to the sin-bin in the closing stages after conceding successive scrum penalties and Lancaster admitted England must be smarter at managing the officials.
"We have to better at managing the interpretations of the referee," he added.
"We want a good scrum and clean ball. It is up to us to adapt."
Wood echoed his coach's sentiments and admitted that it was difficult to take control of the match.
"We were all individually guilty of one error each and that led to a lot of frustration and a very static start from us, which was disappointing," he said.
"We knew what to expect and we were a few percent off and it allowed them into the game. They had some physical individual and it gave them a foothold.
"We rallied under the posts and tried to simplify our game. We went back to our drive. we talked about working them hard and we had our rewards from that.
"It is a shame we didn't manage to the continue that after half-time."