Injuries to captain Vincent Kompany, Micah Richards, Gael Clichy and new signing Martin Demichelis have left manager Manuel Pellegrini short on options at the back, with midfielder Javi Garcia being forced to fill in during recent weeks.
City have kept back-to-back clean sheets in the Premier League, but Nastasic knows that there is still work to be done on the makeshift back four.
"We made some changes in the team - in defence we played Javi and me. It was the first time and I think we did a good job, but I think I can play much better," the 20-year old said after Saturday's 0-0 draw at Stoke.
"In defence, it is difficult when we change the partnership every game. We did a good job but we need to play a lot of games, then it is looking better and better.
"For every team it is difficult to play here, for us as well. After the national team it is difficult because players are very tired.
"The manager is looking at who is fit for the game. We played with six different players. I think we played well but we need to improve and score a goal when we have the chance.
"This point is good for us and now we are looking forward."