I don't know if i believe the reports that he isnt for sale, but i was surprised that he Started against Madrid, especially since he was coming back from injury, maybe Sarri liked what he saw in the limited training time with him. We know that Sarri is methodical in tactics and there are clear patterns that he wants players to do, so maybe Sami's intelligence allows him to adjust to that style quicker, i don't really know.
Also didn't realise him and Matuidi were only a few days apart in age, thought Sami was a few years older, i rate Matuidi more as a player if you put him in alot of other squads, but with Sarri Sami probably fits better, both should go though.