and why his shoulder continues to get injured every 2 months or so for the past 2 years
That makes me not put much stock into his reply. Sheets had back surgery and either didn't or couldn't condition himself properly afterwards. He came into spring training in 2005 out of shape basically. He pulled a muscle in his back during the season.
In 2006 he came back and tried to pitch and never felt fully comfortable. He tore some scar tissue from the surgery which is completely normal and had a small setback. He then pulled a chest muscle most likely because he had modified his delivery because he had the discomfort which again is pretty normal.
In June of 2006 he started throwing bullpen sessions etc and since then has been fully healthy. In fact in Sept he retired the first 18 batters in a game, that is not the sign of an injured pitcher. He went down in the end of August of 2005 and has been healthy since June of 2006. That is less than a year of injuries and he's been healthy for 10 months now. Sounds like that link didn't do their research or had an agenda.
That all was the first major injury he had in his 1088 IP of major league ball and yet he's lumped into the Prior/Wood category for some reason.