Hopefully this turns out to be true but Milan needed to rely on him less and less already this season,,, not more. So I do not think he is a big factor for them at Serie A on the pitch, they just need him in key moments and at training where his fitness may not have such a big impact as they are now paying him for his experience (I believe).
Of course, I also don't expect all Milan players to depend on Ibrahimovic. However, to reach the championship title, IMO, Milan must have Ibrahimovic at least 50% of the remaining matches, its mean Ibra must be played. Unless, in the winter transfer window there are younger and more qualified forwards, then, even though Ibra does not play, Milan still has a striker to score more goals.