class ManagerBridge : ScriptableObject
{
    public Manager instance { get; set }
}

class ManagerUser : MonoBehaviour
{
   public ManagerBridge manager; // filled in automatically in Unity

   public void Update()
   {
        if(manager.instance != null)
            manager.DoSomething();
   }
}

public class Manager : MonoBehaviour
{
    public ManagerBridge bridge;

    public void Awake() // called when the manager is created by Unity
    {
          bridge.instance = this;
    }

    public void DoSomething() { }
}