People that understand the business will play a vital role on your ERP implementation project. To ensure they are able to commit to the project they must be backfilled early
Subject Matter Experts (SME's) play important roles across several workstream through out the lifecycle of the project. They'll be heavily involve with requirements gathering, design, testing, business change, data migration, deployment planning and more.
To set the up for success you must backfill early enough that there temporary replacement can get onboarded ,up to speed and be effective in post before the SME is really needed on the programme.
There is of course a cost implication of doing this so it is important to ensure your project finance estimates include this dual resourcing costs.
You should also consider preparing for the SME's to not return to their original posts. They will build up new knowledge and skills throughout the project and may be able to add value else where in the ERP eco system after the project