Why are people (well, one person mainly) suggesting that "literally the whole stadium", "61,000 people" and so on were booing Sánchez? Obviously I was only watching on TV but it didn't sound remotely like that to me, something that a number of those present have confirmed.
In a vaguely similar vein, whenever a number of people are offered a binary choice - boo/don't boo, leave/remain etc - it is simplistic to suggest that all those picking one option over the other had the single same reason for doing so. All those who voted for Brexit were racist Little Englanders, everyone booing Sánchez thought he was a cunt and so on. The truth is that there may have been a number of reasons that caused a person to join in on Saturday - frustration at the substitution, frustration at the team, frustration at Levy/ENIC, frustration at Stellini being given the job/still being there among others.
Or simply that the person next to them was booing for whatever reason and they joined in without thinking too much as to why.
Back in the day I sometimes sung "You're gonna get your fucking head kicked in" without having the slightest intention of joining in with any potential kicking in of fucking heads, hating the very idea of it in fact, simply because everyone around me was screaming it and I was a lot younger and more stupid. Many of those others were probably in a similar situation, similarly unlikely to get involved with any fucking head kicking-in, but still they sang as loud as they could and pointed menacingly at the away support.
And yes, a load were booing because they thought that Sánchez was shit and yes, I suppose it's plausible that one or two were vile bigots whose hatred was motivated by Sánchez's skin colour but to suggest that "61,000 people" or even 100% of the booers were essentially racists is idiotic, offensive bollocks.