Probably a strain that just causes him too much discomfort to play with.. but crappy he couldn't finish out a strong year with only a few games left. He'll be back ready to go next year.. Rest well gardecky!
Based on this quote, you appear to be correct of a ligament sprain vs. tear.
"With Dennis, he's such a tough guy, we were kind of exploring every option to get him out there," Kingsbury said. "It doesn't appear it'll work out so he'll be put on IR."