The Wallabies were defeated 20-13 to launch the QBE Internationals at Twickenham in triumphant fashion, but, despite the character on show, the performance on Saturday left much to be desired.
Lancaster accepts there were shortcomings, but reckons the result has given England some breathing space.
"I always feel the pressure of winning as England head coach, there's no doubt about that," he said.
"We've won eight out of our last nine games now, but I felt it here because I knew it builds some momentum and allows the team some space to grow.
"But it means nothing if we don't turn up and perform and win next weekend. It's a good start, but we're not getting carried away."
Lancaster will be assessing his midfield options closely after the centre partnership of Billy Twelvetrees and Joel Tomkins disappointed within a disjointed back line.
Twelvetrees, who was flattened by Matt Toomua for Australia's try, could make way for Luther Burrell or Kyle Eastmond after struggling against the Wallabies.
England have viewed the Gloucester centre as a key part of their midfield plans, but Burrell's explosive form for Northampton demands his participation at some point this autumn.
Lancaster insists he has the scope to make changes at centre, where injuries to Brad Barritt and Manu Tuilagi are testing his depth in the position.
"I'll be looking at whole side in terms of selection. You have to build and win the next game," he said.
"Both Kyle and Luther are genuine options - Luther on form, certainly."
Prop Alex Corbisiero and second row Geoff Parling are expected to return from a knee injury and concussion respectively against Argentina.
- Sports & Recreation
- Luther Burrell