Portugal’s No.8 Thibaut De Freitas and David Wallis are in the back row for Sunday’s match (Monday AEST) after being on the bench for the 18-18 draw against Georgia last time out.
Martim Bello returns to the second row in place of Jose Madeira and David Costa makes his tournament debut.
“Some players were tired,” Lajeske told reporters on Friday. “It’s difficult to play three games in a row at this level. We’re not used to that and we need to refresh.”
Nuno Souza Guedes retains his place at full-back, despite missing a late penalty against Georgia that would have given Portugal their first World Cup win.
After stacking the bench with six forwards to face Georgia’s power forward last week, Lajeskit got an extra back for his substitutes for Sunday’s game.
Portugal will need to beat Australia and Fiji to have a chance of progressing to the quarter-finals, which will be difficult despite their impressive performances so far.
“We know we’re not as strong as the Australian team, but far from it. But we need to maintain a good balance between strength and the way we can move,” Lajeske said.
Portugal have caught the eye in the tournament, such has been the woeful form of Australia, there is a school of thought the Wallabies could be in.
“It’s unbelievable. I’m sure you wouldn’t have asked this question at the beginning of the competition, and now everyone is asking,” Lajeske said.
“They are two-time world champions. They are a very strong team with very good players.
“We just have to stay focused on what we can do and not think we have a good chance of winning. We have to compete like we did against Wales and we will see what happens.
“If we’re so lucky, we might have a little luck.”
Nuno Souza Guedes, Rafael Storti, Pedro Bettencourt, Tomas Appleton (captain), Rodrigo Marta, Jeronimo Portela, Samuel Márquez, Thibault de Freitas, Nicolas Martins, David Wallis, Martim Bello, Jose Madeira, Diogo Haasi Ferreira, Mike Tadjer, David Costa . .
Subs: Francisco Fernandez, Duarte Diniz, Francisco Bruno, Stevie Cerqueira, Rafael Simoes, Joao Bello, Joris Moura, Manuel Cardoso Pinto.