Medical staff probably weighed the chance to re-injure the knee if he played. Even as a DH, it works the knees, especially if he would have to slide into a base or something, which is how he first injured it in the minors IIRC. Like I said before, it looked to me like he was running a bit gingerly Saturday night. Better to save that "have to play" mode to later in the season when hopefully we're in a playoff hunt and it is more worth the risk to the player. With two days rest, maybe we get him for an extended time playing.