What is the workaround for extending an offer to a candidate with terminated Hiring Manager?

Summary:

We recently had a case where the parent position's incumbent left the company after a requisition was created using that position. When we got to the offer phase for our chosen candidate, the "Hiring Manager" field was blank. Because we use position sync, that field is not editable, and it would not allow us to move forward with extending the offer. We ended up having to move the candidate to a different req in order to extend an offer to him.

My question is-What is the process on being able to move forward with the offer on the existing req without having to move the candidate to another req? Somehow, the Hiring Manager field needs to get updated.