19-5 is correlation, not causation. It may just means that he gets yanked quickly when he sucks, and so that doesn't appear in the loss column. Or maybe that the vast majority of his minutes come only with the starters, so he's benefitting from their play rather than vice-versa. Who knows.
Regardless, he hasn't been nearly as effective after coming back from the injury as he was before, so season-ling stats in his case are rather worthless. Whether his shoulder is still bothering him, it is a confidence thing, or just a statistical anomaly doesn't really matter.