Could we have a sentence please? What kind of context are you thinking of?
I must have written that while still asleep.
It is "possible", rather than "probable", in these examples. It sounds quite tentative to me.
In the first example it means either of two things are possible. I suppose it may mean that there are other possibilities too.
A more complete sentence/context would be helpful.
1. This person thinks there are two possible interpretations. It sounds as though they are reasonably sure about it, more sure than if they were to say "probably". It sounds as though the doubt indicated by "could" is merely doubt as to which of the two possibilities is the one meant in your context.
2. Without having seen the context I can't be sure why they are using "could". It may be that the writer is expressing his opinion as to who are the likely candidates, those who are possible candidates. It seems to be a little less sure than "probably". They can't all replace the goalkeeper, so none of them is "the probable replacement" for deGea.