I expect that McClellan had physicals before returning to practice. I presume that he had mild run-throughs before he returned to practice (this happened with serval players earlier in the year. They were marked as DNP, when it was clear that they did a bit of work.
Belichick did his due diligence. He thought that McClellan was ready to return so he activated him from IR. We got the 2 week period where he didn't you t against the 53 man limit. Within that time, McClellan was injured again, or he has a relapse of the former injury.
It does seem quite possible for an injured player who hasn't done work in many weeks would be more likely than others to be injured. **** happens.
BTW, we can talk about how Valentine or Mitchell are ready to be major contributors after a couple of weeks of practice, and that we were screwed. I'd settle for ONE of them being healthy enough to play and contribute.