I looked at Purdue's and Michigan's remaining schedules, and there's no way to make up the game without rescheduling other games. Plus, Michigan also missed a game against M$U, and if they could only reschedule one, it would take precedence since it was postponed first and is also a rivalry game.
Anytime Purdue has more than a three day break between games, UM already has one scheduled. So, either Purdue or UM would have to play on consecutive nights, and I don't see that happening.
Assuming they aren't rescheduled, they should be UM forfeits, not no contests. Michigan backed in to a B1G title last year based on winning percentage as opposed to overall conference record (they played three fewer B1G games than Illinois), and while that won't happen this year, Purdue and M$U shouldn't be punished just because UM got COVID.