In 1983, sophomore QB Jim Harbaugh got hurt in the fifth game of the season. Bo didn't have a suitable replacement. Michigan struggled to 6-6 record
That was actually 1984. Russ Rein and Chris Zurbrugg (sp) were the non-suitable replacements, although Zurbrugg came around a little by the end of the season.
1984 was also the last time Michigan was shut out, 26-0 at Iowa. The game was actually close into the 4th quarter and Michigan was driving inside the Iowa red zone, but an interception (the non-suitable replacement thing again) and a long return sealed the deal.
Harbaugh got healthy in 1985 and Michigan bounced way back to finish 2nd in the Nation in the AP, beating Nebraska in the Fiesta bowl.