I'm not entirely sure he's the answer but to be fair to Hummels he has played 30 games or above for the last 5 years. He's struggled this year but then he had an exhausting summer off the back of a long season.
Conversely, Rafael had the summer off and has made above 30 appearances once in his 7 years at the club.
Rafael has 167 apperances in 7 years to Hummels 226. That's 2 more seasons worth of football under his belt in 7 years.
If you look at 13/14 and this season Hummels has suffered from a lot of injuries though. I checked it out a few days ago, he's missed a third of Bundesliga games through injury. So you can see why it's a bit concerning when you're about to make him the general in defense. Granted before then he didn't really have any issues, but 2 years is a long time in football.