There is a very good reason for this at least. In the past Jones' fights were called off because of drug abuse and even crimes that would have resulted in lessor-known fighters being simply kicked off the roster. UFC and Dana White love money more than they like anything else though, so Jones, like a few other people in the UFC, was given a pass.
This time though, Jones has torn a pectoral muscle off of a bone and well, that's about as good of an excuse as anyone could have for missing a big fight.
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The major problem here, and probably a source of great ire for those that have already bought tickets, is that the main fights are the main reasons why people buy the tickets in the first place and since this fight is taking place in New York City, you can be guaranteed that these tickets were extremely expensive. I don't know if UFC offers refunds in these situations but I'd be willing to bet that if they do, they make the process so complicated that very few people would actually pursue it.
The main event is now Jiri Prochazka and Alex Pereira and I guess people are still excited about that one, but it just doesn't carry the same headline as Jones vs Stipe. The heavyweight fight has been replaced with Sergei Pavlovich and Tom Aspinall and even though I am an avid fan of UFC, I had to look up who these people even are.
The worst news about this is that the surgery that Jones has to undergo to repair the torn muscle is going to take an estimated 8 months for recovery and probably longer than that because you don't just get the go-ahead from the doctors and then immediately step back into the octagon. I reckon it will be a year before we even see this fight.