The Blues are currently at the top of the table with 16 points from six matches, and have yet to lose in the league this season.
"We need to improve," Di Matteo told talkSPORT. "This team was put together not long ago and it takes a bit of time [to get things right].
"I think we're pleased with the start in our Premier League campaign but we're looking to improve as well. I think Man United and Man City are still the favourites and City are the champions so it is normal that everyone will be looking to chase them.
"We had a big gap [between Chelsea and Man City] last season and we want to improve on
that this season."
Chelsea finished sixth in the Premier League last season.