I've had two isolated lateral meniscus tears (meniscus repair and meniscus removal), and they have never slowed me down. Granted I'm not running around like Rose does, but I was playing soccer at the college level. A meniscus is there to act as a shock absorber and basically prevent you from getting arthritis. If you have a meniscus repair (basically sew the torn piece back), it can take you around 5-8 months to come back from. If you have a meniscus removal (where they clip the piece that is torn out), it can take you around 3-5 weeks to come back from. Ron Artest had a meniscus removal and was back 12 days after his surgery which was considered very fast, but it goes to show that there are worse knee injuries to be had.
It's very rare to not tear your ACL when you tear your meniscus (they call this an isolated meniscus tear), so in that sense, Rose was very fortunate, lucky, in-shape, whatever you want to call it. I don't think this type of injury is all that serious when it comes to his performance. Yeah, it sucks that he's had 3 knee injuries, but again, when you look at this injury in isolation, it's a rather easy injury to come back from.
I'm not a doctor. This is just me sharing my experiences, so if a doctor wants to come in and refute anything I have said, be my guest...