he has virtually no lift in his legs, and having been suffering through a back injury for the past few months, it effects everything in your core. Your esophagus, everything, tightens up to protect the injured area, which effects your ability to stretch, twist, and pivot. Which are life and death movements if you're injured, and playing ball at breakneck speed.
With some back injuries, there's no risk of re-injuring it. It just kind of stays stiff, and all it needs is some gold ol' rancho relaxo to get better. which is suspect is the case with Amir. If it was something debilitating involving any sort of disc or nerve trouble, he'd be on the shelf indefinitely.