Chelsea, who welcome former boss Jose Mourinho back to the Stamford Bridge dugout, have been widely tipped to take advantage of the managerial changes at their title rivals, most notably Sir Alex Ferguson's departure from United after 26 years in charge.
"It is very difficult to predict what is going to happen before the season has even started," he told reporters.
"They are all great teams who have kept their players together and improved them by buying some more players.
"Although they have new managers, the teams are the same, and there is new motivation with a new manager, so they can be more dangerous."
Chelsea are currently on a pre-season tour of Asia, preparing for their Premier League opener against Hull City on August 18.