Theoretically, yes, that's possible. Olympic athletes do it all the time, drop out of school to train fulltime, then return to their college programs without losing any eligibility (though they do lose a year of eligibility completion time). Whether the NCAA allows it specifically for injury rehab or other cases, I'm really not sure. Practically speaking, I don't think it makes much sense. The player would delay receiving his degree and would be free to transfer elsewhere without sitting out once fully rehabbed.