From Will Carrol's newsletter...
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Javier Baez has an SI joint issue and his season is likely over. The sacroiliac joint is one of those important and nearly immobile intersections where we don’t really think of them as joints. Yes, it’s two bones connecting via ligament, but the placement, movement, and function are so important to almost everything, it has to be taken extremely seriously. Baez isn’t going to die, but the pain can be such that it might make him want to.
This is similar, but not identical, to the issue Baez had earlier this year, which was focused on the lumbar spine. There’s some question about physical changes in that area, which could need surgical fixes. None of them are great for an athletic career, especially someone who relies so much on lateral and linear quickness. Is Baez’s career threatened by this? Yes, especially with three years left on his big deal and declining performance since coming to Detroit. We’ll have to see what the doctors decide is best for Baez and how the Tigers will address this in the off-season. Trey Sweeney will fill in for now and could be the longer-term solution with most of the Tigers top prospects in the low minors."