Kind of hard to tell based on the limited information they're giving out. Based on the tweet saying cartilage damage but no structural damage, that would lead me to think 1 of 2 injuries: either a meniscus, as mentioned by others, or significant damage to the articular cartilage.
If it's cartilage and they had to do a micro- fracture repair, you're looking at 6-8 months.
If it's meniscus there's a large time range. It's most likely not a menisectomy (removal of the torn portion) as that is only a 3-6 week recovery. Beyond that there are at least 2 different kinds of repair. One takes at least 8 weeks, the other is closer to 6 months. Severity, location and type if tear would determine which procedure was done.
And then again, maybe it's something totally different and they are keeping it quiet. Definitely a bummer either way.