Besides, audiences likely won’t know exactly how these characters fit in to the larger MCU (or who Stallone will be playing), until the film hits theaters.
That’s not to say Marvel couldn’t have found a way around this, but perhaps it means Russell and Stallone are being saved for a potential Guardians of the Galaxy 3.
This fits in with Gunn’s previous comments about Stallone’s role in the film, when he emphasized that although it is not a large role, it is a vital one.