While this definitely sucks. I don't really think it effects the Jazz much long term or short term. They were not a championship contender yet. They are still probably good enough to make a run at getting into the playoffs even if they are and were unlikely to make it.
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If Exum is out long term, It means Trey gets his shot to show if he's a starting PG or not. Neto and Cotton get to show if they can be backups or not. Exum's development on the court is going to be put on hold for a while, but I don't believe it will be stunted completely no matter what this injury is.
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To the doomsayers saying he'll never be the same guy that seems really unlikely for a guy his age with the current medical available. Guys recover from ACL tears all the time. And that is pretty much worst case scenario here.
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FWIW I've read about 5 total posts in this thread. So this is just my initial reaction based on the video, news reports and those few posts.