Live pitching is different than a machine. Fielding is different than running. Throwing short distances is different than long distances.
All pose different stresses on different parts of the body. And there's a possibility that there's still some lingering issues that they want cleared up before he does anything else.
Likely because they're running him through exercises and drills while gradually increasing the intensity. They need to see how he responds to that before setting any sort of time table.