Oh Carlos B. As much as we all know you are better at 80 percent than Angel Pagan or Jeremy Reed at 100 percent, do not rush back this year at the possible risk of your career.

The main issue is that there is so little cartilidge left that he’s suffering bruises. The surgery to repair that would be microfracture surgery, which Basketball players like Penny Hardaway and Amare Stoudamire have had. Sometimes it works, sometimes the joint never heals. It’s fairly serious, and had ended a good number of athletes careers.

From March 2008:
Now it’s the issue that’s most problematic for Carlos Beltran as he continues to struggle a bit after minor bilateral knee surgeries. Beltran reports that he feels the most discomfort when he’s stopping, and doesn’t “feel stable” when he’s starting either
May 09
A greater matter of concern is Beltran’s situation. He played through a knee injury that has been diagnosed as a bruise to his tibial plateau. That’s the ‘top’ of the bone, where most would say it’s the knee, and where the load from the body is born. Yes, that means that the cushioning inside failed for some reason, and that the bruise was probably caused by the femur slamming into it enough to cause damage. That’s very bad in the long term, especially if Beltran’s meniscus has a deficit. (I couldn’t find if he’s had a previous meniscectomy, though I’m sure you’ll chirp in comments.) If there’s no cushioning, this will continue to be a problem, with grinding as well as the occasional traumatic injury such as a bruise or worse, a fracture. I think that we have to take a year or two off the back end of Beltran’s career unless he finds something like Synvisc or microfracture to reduce some of the damage. For now, he’ll miss a couple of games after a cortisone injection, but the team thinks he’ll be able to avoid the DL.
June 22
The other big factor now is the continued problem with Beltran and his chronically sore knees. He’s is headed for an MRI on Monday, but he played through the weekend… and played well! Beltran’s knees have lost a lot of cushion, so much so that he’s occasionally had bruising where the femur hits the tibial plateau. That’s very bad long term, and not a good sign in the short term either. There has been discussion of possible fixes like Synvisc, or even a more radical procedure (video link). Beltran has two more years to go on his Mets contract, but it includes a lot of deferred money. He could be a good fit for a deal to an AL club this offseason.
June 23
The MRI on Beltran showed that the bone bruise in his knee was getting bigger. This led to two things. First, the Mets put him on the DL, and then Joel Sherman tweeted that it made him think of this classic. So we get some good with the bad. As I stated yesterday, Beltran’s knee has been problematic for a while, leading some to wonder if he can stay in center field or if he can even stay in the game long-term. I mentioned a meniscal transplant, but actually the most likely off-season scenario might be for Beltran to have microfracture surgery. This is precisely the type of situation where the operation does the most good, but the results in baseball have been mixed, and there’s no good comparable player for a plus defender like Beltran. The Mets will now focus on his rehab and just hope that maintenance and rest keep him productive. The team expects him back just after the All-Star break.
June 30
Everything I’ve read about Beltran heading to Vail has had a decidedly negative aspect to it. Yes, having Beltran in the home of microfracture surgery is certainly not something you’d want if given a choice, but the knee problem was already extant; worse, it was already recurrent. When he went onto the DL, this was always a possibility. What’s making this so negative is that people are speaking about it as if it’s an inevitability rather than just a possibility. Both Beltran and (to some extent) the Mets are speaking with some of the best orthopedic surgeons in the world about a procedure that could help one of their best players get back on the field. There’s not much of a track record of microfracture procedures in baseball—the demands aren’t the same as in basketball or football—but it has been reasonably successful. The questions now become if microfracture would in fact help Beltran, and then whether or not it can be delayed until the offseason.

I apologize if this was too much info and again everything comes from Will Carroll via free content at baseballprospectus.com