The story may be fiction, but there are certainly compelling reasons why Jose would be open to selling both players. Not much needs to be said about Mata, as Mourinho has already seen fit to sell him once (to a rival, nonetheless, which probably tells a story in and of itself). Mata can be a clever player, but he's a luxury one. He's clearly not a winger. Moreover, he simply doesn't possess either the all-around game or the physical attributes that Mourinho demands of a number 10. As likeable as he is, he could logically be deemed surplus to requirements. Blind, on the other hand, is a classic "jack of all trades, master of none". Personally, I think he's done an admirable job this season, playing the entire year out of position. He was exposed at times, but deserves a lot of credit for the job he did for the team. Moving forward, however, I think most would agree that Blind is not an ideal centre back. He may be an option at left back, but we have Shaw, Borthwick-Jackson and Rojo who can play there as well. As a midfielder, again, he lacks pace, power and height. He can put a shift in, but is far from top class.
One thing I would also note, is that we're not talking about two vital cogs in a championship calibre team. We were pretty dreadful for the vast majority of the season. A massive amount of the blame rightfully goes to Van Gaal and his staff, but the players aren't blameless either. We need to get better. If Jose sells players like Mata and Blind, he's not going to replace them with lesser players. If he moves them on, it's because he intends to replace them with better players. It's easy to overrate players like Mata and Blind. I like both of them, but if it means improving the squad (and our chances of winning), then they should go.
