I have actually had some really positive experiences with this so far (it may just be fluke though):
I simmed a season with Joel Embied, and he didn't progress during the offseason having missed the season with his real life injury.
I had Avery Bradley go up a one OVR points during the first season, but then got a serious injury. When back from injury he had dropped 3 OVR points (-2 from starting position), with his physical stats affected. He then didn't progress during that offseason. However he has now bumped back up one OVR point midway through my second season with him playing big minutes.
Jared Sullinger had a less serious injury. His in season training bump from the season got removed, but that was it. He didn't progress during the offseason. He has also bumped up on OVR point in the second season.