The overreaction by the fans shows how they feel for the young star player, they want to see him play because they are hopeful that Mavericks will be successful this season in making to the playoffs, and of course when they are on the playoffs they will not be just a heavy underdog as Luka is doing magical things that could give their team a possibility to upset a heavy favored teams.
I feel that but have they not played basketball before or seen other NBA players sprained their ankles? That is a very normal injury to receive such a reaction.
Since his in the rehabilitation for his injury, I think fans are a bit worried if he still can make it to the MVP race or he will be left behind by his competitors. Since Doncic will still be nursing for his injury, his competitors will be ahead of him in terms of statistical points and that would be hard for him to coup up. Anyways, as long as he can return healthy, the Dallas would be regaining their morale for their goals.
He is missing a few games, he is still very much in the race. We are not even half the regular season. His competitors will also miss some games at some point during the season.