How much notice should an employee receive if the company is lowering their wage?

Good afternoon, 
We have an employee who started mid October 2020.
Manager wants to lower their wage (doesn’t want to terminate) as they are unable to conduct the work; they are unproductive, struggling to complete their duties.  Manager thinks it is better to have someone rather than have no one in that position.
How much notice should an employee receive if the company is lowering their wage?