Last season, City won the EPL title and finished top of the Premier League with a whopping 100 points, 19 more than second-place United.
United's record goalscorer said: "I think it will be hard after the season they had past year".
Puma will take over from Nike at Manchester City and will also provide kit for La Liga side Girona, Australian outfit Melbourne City, Japan's Yokohama F Marinos and Uruguayan outfit CA Torque.
The difference this season is that the Merseysiders have greater strength in depth with the addition of Fabinho and Naby Keita adding increased options in midfield.More news: 'Impossible' to make Mars like Earth with current technology - NASA study
The Puma deal is a significant uplift for Man City who were getting an estimated £20 million a year under the old Nike deal. After that, Maurizio Sarri and Unai Emery must answer questions before Chelsea and Arsenal can be confident of overhauling a discordant United and a Spurs side investing only in a new home.
Meanwhile their neighbours and reigning champions, Manchester City, have only acquired Riyad Mahrez and Pep Guardiola has already mentioned that a player to back up Fernandinho would be more than welcome.
All in all, there's never been a better time to join BT Sport as it is set to be our biggest and best season yet.
Writing on Twitter, Collymore said as he predicted the top four: "City".