Despite winning the Champions League and FA Cup, Chelsea finished a disappointing sixth last season.
Since then interim manager Roberto Di Matteo has been confirmed as permanent boss at Stamford Bridge and has already started reshaping his squad.
The signings he has made will make the Blues more of a threat next season, according to Mancini.
"For sure [they will be a bigger threat]," said Mancini.
"They have bought young players, but also very good players."
City face Chelsea in Sunday's Community Shield and the manager is hoping for a win.
Mancini added: "It is important we try to win because we have not won this cup for along time and we want to start the season with a victory.
"We know it will not be easy. Chelsea are a strong team."
Mancini also confirmed that neither Mario Balotelli or David Silva will start the game, which will be held at Villa Park due to London Olympics.
The game kicks off at 1.30pm tomorrow.