It has become somewhat fashionable to blame Mark Connor for Eric Hurley's shoulder problems - a torn rotator cuff and frayed labrum. Without a full blown analysis of Hurley's arm action, though, it is also hard to say with certainty that his mechanics were responsible.
Given the nature of these injuries, though, Hurley's mechanics and hamstring are far more likely than Connor's teachings to be the cause of Hurley's shoulder injuries.
ANATOMY
Like any soft tissue in the body, rotator cuff muscles and tendons are torn gradually over time as stress creates micro-tears that build up and compound. There are exceptions, of course, but most of them involve severe external trauma like violent collisions and power lifting.
In pitching, the rotator cuff contracts most powerfully during the deceleration phase as it tries to keep the humerus from twisting and flying out of socket. When the arm moves across the body, the head of the humerus becomes an obstacle to this contraction. This forces the muscles to contract "around a corner" which adds more tension to the muscle than it can create on its own.
A frayed labrum is an early stage SLAP (superior labrum from anterior to posterior) lesion. Later stage SLAP lesions are commonly referred to simply as "torn labrums". The lesions are caused by the compressive force and friction created when the long head of the biceps brachii contracts and pulls directly on the glenoid labrum in an unnatural manner.
Certain arm actions, most notably transverse hyperabduction of the shoulder (scap-loading), can position the head of the humerus as an obstacle to the contraction of the biceps creating extra tension on the labrum where the long head of the biceps attaches.
Since part of the long head of the biceps merges with the labrum, SLAP lesions can sometimes be misdiagnosed as biceps tendinitis. This is was the reported initial diagnosis for Hurley's shoulder injury on July 30, just three days after his final start of 2008. On August 1, the Rangers reported that it was, in fact, shoulder soreness.
ATTACK OF THE HAMSTRING
Hurley was cruising along fairly well before he injured the hamstring of his left leg - his landing leg.
The hamstring of the landing leg experiences an eccentric contraction as the upper body moves forward over the waist. A negative change in the muscle's flexibility can decrease the amount of trunk flexion and/or shoulder rotation that occurs during a pitch. Since the body is less engaged in the deceleration of the arm, the shoulder handles more of the load than it would with normal hamstring flexibility.
Limited trunk flexion or shoulder rotation can cause the throwing shoulder's forward movement to stop early, even though the arm tries to continue moving toward the plate. The force of this action slings the arm across the body and moves the head of the humerus into the path of the muscular contraction as described above.
Hamstrings are notoriously slow-healing muscles, and flexibility can be compromised for a long period even after the muscle is fully functional.
Hamstring injuries will not always lead to shoulder injuries, but they represent a huge risk factor for someone already dealing with a weakened shoulder.
WAS HURLEY'S SHOULDER ALREADY WEAKENED?
The answer to this question simply has to be, "Yes."
In 234 minor league innings over the last two seasons, Hurley worked primarily with Rick Adair, Terry Clark, and Andy Hawkins - one of whom has one of the greatest mustaches in baseball. All three coaches are extremely well regarded; none of them is Mark Connor.
Mark Connor was Hurley's primary pitching coach for about 32 innings, all in 2009. (Hurley had a 7.1-inning rehab start in Frisco near the end of that 32-inning span.) 32 innings is simply not enough to tear the healthy rotator cuff of a professional pitcher - someone whose rotator cuff should be exceptionally strong and well conditioned.
Barring severe external trauma, his shoulder must have been compromised before reaching the Majors and long before he hurt his hamstring.
SUMMARY
Hurley likely began damaging his shoulder well before his injuries became apparent. One can argue about the inevitability of a major tear, but excluding an external traumatic event, Hurley's mechanics are the most likely cause of the injuries.
When his hamstring started giving him trouble, his body compensated for that injury, effectively placing more (too much) stress on a rotator cuff that was, in all likelihood, already damaged.
Rotator cuffs simply don't tear suddenly enough to blame Connor for the injury.
Why Hurley was allowed to start that last game (and remain in it for as long as he was) is a different matter entirely.