Per the Spurs official press release:
Kawhi Leonard continues a rehabilitation program for right quadriceps tendinopathy. He is expected to miss the 2017 preseason. A timeline for his return to the court will be determined at a later date.
As I’ve literally never heard of that condition before, I’d guess it’s a mild injury — maybe a precursor to full-on tendonitis. (If you have info about this specific condition, message me on Twitter or drop the info in the comments.)
It appears to be classified as overuse, so we’ll keep you updated on how this develops.