So here is a question. If the injury is old and surgery was planned... why did he extend and stay with the show? Does production have to pay for his medical leave? Or when they say that Santino is filling in.. does that mean he just gets Gavin's salary while he is out? Or is it just a additional salaried position? How do these things work?
With back issues, you never know how long the recovery period will be with resting, physical therapy and exercising. Why wouldn't he stay with the show? It's still a huge hit, and he won The Tony for it.
His medical insurance would cover it. Just like with any job when someone is out on disability leave, he'll get paid, probably not full salary, but he'll get paid.
As far as Santino's salary, his agent would handle that, and I'm sure he's getting a standard salary for a supporting role.