I've got a couple of senarios...
A. Just push the Ohio State/ Michigan game back to next Saturday, Dec. 19th. It really doesn't matter who the Buckeyes play in the big ten championship, it's irrelevant. Northwestern is #14 so it's not even a real championship game as far as the playoffs go. OSU/ Michigan next week, Buckeyes get their 6th game in and the rivalry continues.
or...if somehow the Buckeyes don't get into the playoff, have the Ohio State / Michigan game on New Year's Day, what better matchup. Play in Indianapolis so the weather isn't a factor.