It's not schneiderlin specifically that got booed but the sub that was made.
The idea of being 3 up at home and the better team and we are looking to defend it? The idea that we can go out and sign a actual striker and yet again he can't get on the pitch but we are told he isn't up to it?
Niasse wasn't up to it, he is the difference between a hopeful finish to the season and a relegation pretty much so far.
The fans want to see player a given a chance. The likes of klassen tosun vlasic etc so when at home and you are making literally defensive subs (2 defenders and a defensive midfielder) then doesn't that represent the current affairs at the club?
I can guarantee the boos were because a lad like tosun wasn't given a chance to even get on the pitch and for me that is completely justified.