I'm still backing Manchester City to win the Champions League title next season, so I have no doubts with them. But of course, some additions player next season are expected as Mahrez and Gundogan are unlikely to stay around.
They have good strength and enough ambition to protect their titles. They can even buy some players to improve their squad in this summer. Pep Guardiola is known as a coach who has never stopped upgrading himself and his tactics.
However, being strong and ambitious, as well as upgrading your squad, won't be enough to protect your crown in Champions League. It depends on other clubs and your luckiness in a new season. Champions League has many knock-out matches after group stage and with many such tough matches, many things can happen. In history only Real Madrid successfully protect their crown.
In order to cover for injuries, every team needs to have good first and second-eleven players. Being an intelligent manager, Pep Guardiola does not always start his starting eleven in every game for Manchester City, and this has been producing positive results for him. He alternated the first and second eleven players in minor matches, but he always fields his strongest squads in tough matches.
Manchester City have one of deepest player resource in Europe and it is good for them in a long season with many big competitions like Premier League and Champions League. They have more choices to flip their debut squads and give their important players more resting time than clubs with less deep player resource. It is very helpful for them but it does not mean they will be able to protect their crown only because of a deep player resource.