It's ok to criticise players when they make mistakes, irrespective of transfer fees/match results. And just because you defend players from criticism doesn't make you a better fan.
Everyone here loves Spurs and wants what's the best for the team. I know Sissoko has improved, but his skill level is quite low compared to the rest. His first touch is not great, he scuffs his lines very often and often makes the wrong decision.
Now in the Madrid game it didn't cost us, but if it was a game where we are drawing 0-0 and he scuffs his lines like what he has done several times already this season, I am sure everyone would jump on him. He's a decent option, but if I had to replace someone from our squad to improve, he's the easiest player to spot.
The one kind of player we are missing is someone like Mane/Hazard = direct and skillful and quick. And between Lamela and Sissoko, one of them have to upgraded to be that kind of player.
Let's see how Lamela is after a year away before that decision is taken. But Lamela gets the benefit of doubt from me because he was really good before his injury and Sissoko has not been anyway near as good as literally everybody else we have (except N'Koudou of course, but we paid 30 million for Sissoko).