They actually aren't rivals in the traditional sense. It's not a longstanding contest and neither claims the other as one (PSU technically claims no rivals; that's one of the meanings of their "#unrivaled" line).
In the more in-the-moment sense, I suppose you could call anyone a rival so long as they consistently stand between you and a championship. That would grow Michigan's rival lost to include PSU and UW. Harbaugh is 2-1 versus each of those (with another PSU match fast approaching).
That'd go for a tentative 6-8 record versus "rivals," which is still bad for Michigan In Maintenance Mode. But of course Michigan isn't yet in maintenance mode. And maybe 6-8 is exactly what you'd expect for a helmet in Rebuilding Mode.
Whether near or far, I'm really looking forward to the time Michigan next returns to maintenance mode.