It all comes down to Clemons, and how well he heals as a person. Sharkhawk has it right; he'll be running and fulling rehabbing in 6 months. It really depends on two things: 1. the quality of rehab, which Clemons will get top notch, and 2. the efford the person puts into rehab. Genetics and how faat he heals will also help, but the effort you put into really is the key. Clemons is a hard worker and does a good job to keep himself in shape; at 31 he hasn't seemed to have lost a step yet.
15 years ago, I tore my ACL in the Marine Corps. I opted for the patellar tendon graft. At the time, the Navy had a very aggressive rehab approach. I was actually doing physical therapy the day after surgery (heel slides). I never wore a brace; the doctor told me he believed that the supporting muscles will atrophy worse relying on a brace, then it also can become a mental crutch. I was also told that you just can't run on it, no matter how good it feels for 6 months, as the graft is still soft in there and could stretch out.
This was military medicine (which I felt was very well done) 15 years ago. It sounds like they have shortened the time you can run through modern medicine. If this guy gets top of the line care, and has the drive, he'll be back. He can be PUP'd for 6 weeks of the regular season and not count against our roster, and his contract was fair to both sides, so he isn't breaking the bank. From a business standpoint, there's no reason to cut a previously productive player like Clem.
Sure, if he doesn't come back the same, he'll be done, but he'll get the opportunity next year to prove it, that I'm sure of.