Junior's game pits his current school against his former school. It is a chance for Junior to bring the two halves of his life together on the basketball court in front of those very people from whom he was escaping. The conflict and the opportunity this presents to Junior is very Shakespearean in nature. The high drama of both physical and emotional conflict was a staple throughout all of the bard's plays. Rarely, in real life, are such opportunities afforded to individuals but in the world of literature and Shakespearean drama they happen quite often.
The reason that Junior says this because Shakespeare's plays are full of surprise twists and conflicts. You have lots of examples of people in those plays being put in situations that are very emotionally charged. Junior is in such situation when Reardan has to play Wellpinit.
Junior has left the reservation to attend school Reardan. This causes conflict between him and the Indians who think that he has abandoned his people. Basketball is a huge deal among Indians today and so playing basketball against the people he has "abandoned" will be an emotional experience for all involved.