I'm not saying you should do anything. You asked if you could cut their pay because they are disabled, the answer is no, that constitutes unlawful discrimination. I agree with you on that but the OP is not suggesting he cut someone's pay because they are disabled. He is stating that he is willing to create a new job for someone who can no longer perform his customary job and the new job has reduced responsibilities. The OP would then pay the individual the rate of pay associated with that job.
I do agree the way to go is for the start with a doctor's note provided by the employee which states clearly what the employee can and cannot do. If the OP has tasks that need doing and is willing to create a position doing those that are within the employee's restrictions, that's perfectly doable (and generous.)
If you have to give them another position with fewer hours because their doctor verifies they can't do the job as it exists, as long as you don't cut their pay, that's fine. That's just not correct. The OP may reduce the employee's rate of pay to an appropriate and fair pay rate for the new position.
To give an extreme example of what you're suggesting, the CEO of a company making $500,000 a year has a stroke and no longer has the mental capacity to do that job but he is capable of performing the janitor duties. You're stating that it's ok to make the CEO the janitor but he still has to be paid $500,000 a year. That's just not correct.