I have high school sports training in recognizing and addressing concussion. The blow or blows to Baer's head, or it getting knocked around, may have happened well before he felt any symptoms. It even may have happened in practice, a previous game or doing some other activity. The activity that led to the concussion may not even have been physical or violent in nature but just a rapid movement of the head, as if he fell backwards or to the side and his head snapped one way or the other.
It's also possible (likely?) he has suffered concussions previously. Many athletes do. If so, he may be more prone to it again.
As much as I'd like to see him play, he shouldn't be rushed back onto the court. The best treatment is rest, limited physical activity.
Sucks for Nicholas. And for the Hawks.