First, I am not sure what "back up Rondo" means - but IF it means "provide support to Rondo" [whoever/whatever that is] then I would suggest ...
There is very little difference, if any.
The first says that he has a continuing, ever-present reluctance to provide support, and that applies to past, present and future Celtics events.
The second says that won't go because, if he goes and only if he goes, he would not be able to provide support. It does not say he has an abiding inability to support. In fact, if he stayed home, Rondo might be able to count on him.
Substitute "is unable to see from those seats" and "wouldn't be able to see from those seats" and you may see the subtle difference?
Maybe someone else can supply a better substitution to clarify the difference?
Student or Learner