At the time of my post, it was publicly stated it was strain.
https://www.si.com/nfl/browns/news/cleveland-browns-deshaun-watson-shoulder-injury-update-before-colts-game
"Watson described the injury as a strain, or “micro-tear,” of his rotator cuff, which he acknowledged was usually a 4-6 week injury. The injury is in his right throwing shoulder, however, which has affected his ability to adequately throw the football with velocity."