De Bruyne is a key player obviusly but i think Manchester City can sub him very well, in fact i really think this injury helped Manchester City thinking in the future of the team. Now they know they can play very good without him, so we can some news in the next summer transfers window.
Kevin has high-level game intelligence. Maybe Manchester City can replace him, but I don't know if they can be as productive as him. I think Manchester City should keep Kevin in their squad for a longer time. They can still benefit from it.