tzill
Lefty 99
It is where you LIVE and/or WORK that determines where you pay taxes. Going to see the doctor is not working. He can have the operation, go 'home' for a month or three, then re-visit the doctor when needed.
And Arizona's top tax rate is 4.54%, so Arizona isn't that bad. But I don't understand why you think that he will be spending extensive time in Arizona, at least in the near future. The 'rehab' doesn't start for months after the operation, and the 'working up to baseball activities' is months after that. He has *at least* 6 months of mostly down-time, with low-impact exercises/stretches.
With pro athletes, it has to do with "duty days." I'm unfamiliar with how much a team would want someone coming off surgery at the training facility to rehab. You're correct a visit to a specialist wouldn't count. Spending time in AZ when the team requires you to be there would be duty days, and pay would be taxed at the AZ rate. I know a fair bit about taxation (it's my field) but very little about rehab.