The Catalans are top of La Liga and won their first Champions League game, but have still been criticised following unrivalled success under Pep Guardiola.
Iniesta, who was recently named the best player in Europe by UEFA, has admitted that continuing that success would always be hard.
"It appears when you win everything, it seems normal and easy, but it is very difficult," he said.
"The challenge is wanting to improve, wanting to keep winning championships."
The 28-year-old also said that the squad misses Guardiola.
He continued: "With him we reached the top and we also enjoyed ourselves a lot. I think that's paramount.
"We enjoyed everything, every workout, every game, we learned a lot and will miss him in that sense."
Iniesta was speaking ahead of Barcelona's trip to Benfica this evening.