City bought star players David Silva, Mario Balotelli, Yaya Toure, Alexander Kolarov, Jerome Boateng and James Milner this summer, as new manager Roberto Mancini set about spending his owners' considerable wealth.
But despite that outlay there have still been mixed performances so far this term, with 11 points from six league matches placing them fourth.
"City is proof of how big-name signings and big spending, almost 100million euros if I'm not mistaken, is not enough to obtain great results," said Motta in an interview with Sky Italia.
"But it's clear that they are a rival that has all of our respect and we will have to be on guard."
Juve, under new coach Luigi Del Neri, are still trying to find their balance and are ninth in Serie A with just two wins in five games.
"We are trying to adapt with the arrival of at least 10 new elements this summer," said Motta.
"To assemble the newcomers takes time.
"Winning helps you win, and allows you to train meeting the expectations of the fans, the club and a great coach like we have."