Thats not a fact, thats your opinion. These are very different things.
You need to be very specific in what "best performer" even means.
In terms of 'which players performed at their absolute best'? I would agree Sissoko is right up there.
In terms of 'Which players played the best football in our team'? Ive already stated many times - Sissoko is probably #7 on that list. No where near the top.
The logic of going for someone like N'Zonzi has been made clear, twice now.
A) I think he is a better footballer than Sissoko
B) I think Sissoko (and Wanyama) are both set to be way down the pecking order, and will only be on the bench/be utility players
C) I think the profit in selling both and replacing with N'Zonzi would be a good strategy, trimming the squad and raising cash to invest.
You might or might not agree with my conclusion, but there is nothing about it that should baffle you.
Further to that, you assert that Sissoko is already performing in the team at a high level. As Ive already stated, I do not believe this is true. I have posted at length in this very thread the ways in which he does not suit the team, is not good enough for it, effectively compromises it... Quite the opposite to your view.
I place very little value on the actions and opinions of international managers, its a silly point to raise.
As for anything that happens, it remains to be seen. We can only predict.
I predict Poch wants us playing football more like seasons past than last, and as a result Sissoko will disappear from the first team, perhaps only to be seen late in games when we are closing matches out and in the early rounds of the League/FA cups.