The likelihood of a back injury severe enough to cancel the fight happening in the very light training days of the weight cut … in not zero but it’s very low.
Either he was severely injured for days/weeks and hoped it miraculously healed in time, or it’s a weight cut issue and he doesn’t want to lose the championship opportunity by being overweight or severely compromised.
All around not a good look with 1 day to go, plus the message doesn’t offer any kind of apology to his opponent, the fans and the UFC.