Myles Jack was reported to have a chondral defect, and the words "microfracture surgery" did start to pop up. He also had a torn meniscus, but opted to have it repaired instead of trimmed, which is what extended his recovery time. We'll see how that goes with time, but in theory, you can fully recovery with a repaired meniscus, whereas a meniscectomy means they've removed some of the meniscus, and you're likely to have continued issues.
The chondral defect issue is another story. The fix is chondroplasty -- they shave and smooth the actual cartilage in the joint, which means some loss of cartilage. (Just like a nose job -- rhinoplasty, that's always made me laugh, like someone's nose is so big that they're comparing it to a rhino, how immature, lol -- except you don't carry 200-300+ pounds on your nose.) Same as a meniscectomy, you're likely to have issues in the future.
I don't know the nature of Mewhort's issue -- whether he injured it abruptly, or if there was an issue that was building up over time, or if it's genetic and degenerative -- nor do I know the extent of it, or the scope of the operation. I'm assuming that it was caused by his on field injury, which could mean that there was significant damage and they had to remove a significant amount of cartilage, but not necessarily.
And of course, loss of cartilage, could lead to microfracture surgery in the future, but that's out of anyone's control at this point. The Colts could protect themselves contractually, but that's another story.
Cartilage and meniscus issues just make me nervous, because unlike ligaments which can fully heal (or close to it; DeJuan Blair is a former NBA player who actually had NO ACL in either knee, a result of surgery when he was in high school), once you have a chondroplasty or meniscectomy, that cartilage or tissue isn't coming back. Your joint is weakened and/or lessened to a certain extent, and always will be. And there's the likelihood of loose bodies as more cartilage is damaged, and it sometimes leads to microfracture, which as you stated is the bane of any athlete's existence.
If the docs say Mewhort is okay, awesome. But the Colts should keep his knees in mind when they do his next contract, and he should probably work on some insurance himself, if he hasn't already.