Richard Keys (who I know is a bit of an idiot) has been saying for months that an offer is on the table for Cavani, since way before Moyes was sacked. I personally think Moyes/Woodward were trying to line him up as they expected van Persie to be off.
He also stated that the Club would have to then try and persuade Cavani to join, as at the time we were struggling to qualify for Europe. Following that, I think there was a story at the end of the season where Cavani said he was staying at PSG. Unless things have progressed since then and given we didn't qualify for Europe, albeit for one season I expect, I don't see this deal happening.
If it does, I'd be surprised and I'm not sure where it'll leave Rooney. Nonetheless, whether Rooney or Cavani are at the Club, I see van Persie as van Gaal's first choice centre forward. Rightly so in my opinion.