If we have an employee whose on the job injury required surgery so he was moved to a different position as he could no longer do his job as a salaried manager, can we change his rate of pay/benefits?

Theoretically, if he simply could not, even with some "reasonable accommodations," do his old job and that's why he was moved, you could alter his pay or benefits. If someone cannot do the core or imporant parts his job due to a medical condition even with accommodations, he could be terminated or laid off: employers don't have to keep employees who can't do their jobs and not required by law to give them different jobs. Since in this case, you could have terminated him entirely, if you voluntarily give him a different job, you can match pay/benefits to the new position.
But we say "technically" because this is a rife area. If he could have kept doing the core or important parts of his job with some reasonable accommodations (changes in rules or procedures or working location, or provision of some assistance device, which is not too expensive and disruptive for the employer), but you moved him because it is more convenient or efficient for you, then reducing his pay or benefits would be illegal disability-based discrimination. Employers must accommodate employee medical conditions when reasonable to do so. So if there is any reasonable chance you could have kept him as a salaried manager, even if you had to make some accommodations, if you reduce his pay or benefits, you will be engaging in illegal discrimination. If at all in doubt, keep his pay/benefits the same.