Design Service Manager Class which uses external services using GUICE

In my application I have a Service Manager that handles all my requests. Now this Service Manager uses external services in order to fulfill some particular kind of requests.

For example,consider below sample code:

 Class ServiceManager{

    private final A a;
    private final B b;

    private ExternalService externalService; 


      @Inject
      public ServiceManager(A a, B b)
      {
                this.a =a;
                this.b =b;
      }

      public void processIncomingRequestUsingExternalService(){...}


      }

Now in order initialize ExternalService which approach is better using GUICE

• Use setter injection, as in future there might be 'n' number of different type of external service come into picture which this Service Manager might want to use.
• Or use constructor injection, as I have used for class A and B objects which are internal class of my application.

Note:- Here ExternalService is a Helper for External Service.